v3.26.1
Risk/Return Detail Data - FIMMFunds-SelectComboPRO - USD ($)
 12 Months Ended 60 Months Ended 120 Months Ended
May 30, 2026
May 30, 2026
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
May 30, 2026
May 30, 2026
FIMMFunds-Class4ComboPRO | Fidelity Investments Money Market Treasury Only Portfolio | Fidelity Investments Money Market Treasury Only Portfolio - Class IV                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.50%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.71%                          
Fee Waiver or Reimbursement [1] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.68%                          
Expense Example, with Redemption, 1 Year $ 69                          
Expense Example, with Redemption, 3 Years 223                          
Expense Example, with Redemption, 5 Years 391                          
Expense Example, with Redemption, 10 Years $ 879                          
Average Annual Return, Label [Optional Text]   Class IV                        
Average Annual Return, Percent   3.63%                     2.72% 1.67%
Annual Return [Percent]     3.63% 4.62% 4.36% 1.08% 0.01% 0.20% 1.54% 1.20% 0.24% 0.01%    
FIMMFunds-Class4ComboPRO | Fidelity Investments Money Market Treasury Only Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Only Portfolio /IV                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Only Portfolio seeks as high a level of current income as is consistent with the security of principal and liquidity.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities. Normally investing in securities whose interest is exempt from state and local income taxes. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.75%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.20%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class4ComboPRO | Fidelity Investments Money Market Treasury Portfolio | Fidelity Investments Money Market Treasury Portfolio - Class IV                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.50%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.71%                          
Fee Waiver or Reimbursement [2] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.68%                          
Expense Example, with Redemption, 1 Year $ 69                          
Expense Example, with Redemption, 3 Years 223                          
Expense Example, with Redemption, 5 Years 391                          
Expense Example, with Redemption, 10 Years $ 879                          
Average Annual Return, Label [Optional Text]   Class IV                        
Average Annual Return, Percent   3.66%                     2.78% 1.71%
Annual Return [Percent]     3.66% 4.63% 4.50% 1.17% 0.01% 0.20% 1.57% 1.21% 0.26% 0.01%    
FIMMFunds-Class4ComboPRO | Fidelity Investments Money Market Treasury Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Portfolio /IV                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Treasury securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.76%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.20%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-SelectComboPRO | Fidelity Investments Money Market Government Portfolio | Fidelity Investments Money Market Government Portfolio - Select Class                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.05%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.26%                          
Fee Waiver or Reimbursement [3] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.23%                          
Expense Example, with Redemption, 1 Year $ 24                          
Expense Example, with Redemption, 3 Years 80                          
Expense Example, with Redemption, 5 Years 142                          
Expense Example, with Redemption, 10 Years $ 327                          
Average Annual Return, Label [Optional Text]   Select Class                        
Average Annual Return, Percent   4.15%                     3.12% 2.04%
Annual Return [Percent]     4.15% 5.12% 4.97% 1.43% 0.01% 0.32% 2.04% 1.67% 0.70% 0.20%    
FIMMFunds-SelectComboPRO | Fidelity Investments Money Market Government Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Government Portfolio /Select                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Government Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities). Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities. U.S. Government securities are high-quality securities issued or guaranteed by the U.S. Treasury or by an agency or instrumentality of the U.S. Government. U.S. Government securities may be backed by the full faith and credit of the U.S. Treasury, the right to borrow from the U.S. Treasury, or the agency or instrumentality issuing or guaranteeing the security. Certain issuers of U.S. Government securities, including Fannie Mae, Freddie Mac, and the Federal Home Loan Banks, are sponsored or chartered by Congress but their securities are neither issued nor guaranteed by the U.S. Treasury. The issuer usually pays a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Government securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.87%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.31%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-SelectComboPRO | Fidelity Investments Money Market Money Market Portfolio | Fidelity Investments Money Market - Money Market Portfolio - Select Class                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.05%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.26%                          
Fee Waiver or Reimbursement [4] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.23%                          
Expense Example, with Redemption, 1 Year $ 24                          
Expense Example, with Redemption, 3 Years 80                          
Expense Example, with Redemption, 5 Years 142                          
Expense Example, with Redemption, 10 Years $ 327                          
Average Annual Return, Label [Optional Text]   Select Class                        
Average Annual Return, Percent   4.20%                     3.20% 2.21%
Annual Return [Percent]     4.20% 5.18% 5.07% 1.64% 0.01% 0.49% 2.20% 1.95% 1.08% 0.43%    
FIMMFunds-SelectComboPRO | Fidelity Investments Money Market Money Market Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Money Market Portfolio /Select                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Money Market Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements. Investing more than 25% of total assets in the financial services industries. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.88%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.33%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-SelectComboPRO | Fidelity Investments Money Market Tax-Exempt Portfolio | Fidelity Investments Money Market Tax Exempt Portfolio - Select Class                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.05%                          
Other Expenses (as a percentage of Assets): 0.08%                          
Expenses (as a percentage of Assets) 0.27%                          
Fee Waiver or Reimbursement [5] (0.04%)                          
Net Expenses (as a percentage of Assets) 0.23%                          
Expense Example, with Redemption, 1 Year $ 24                          
Expense Example, with Redemption, 3 Years 81                          
Expense Example, with Redemption, 5 Years 146                          
Expense Example, with Redemption, 10 Years $ 338                          
Average Annual Return, Label [Optional Text]   Select Class                        
Average Annual Return, Percent   2.56%                     1.98% 1.39%
Annual Return [Percent]     2.56% 3.22% 3.15% 1.02% 0.01% 0.43% 1.31% 1.25% 0.66% 0.32%    
FIMMFunds-SelectComboPRO | Fidelity Investments Money Market Tax-Exempt Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Tax-Exempt Portfolio /Select                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Tax-Exempt Portfolio seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing in municipal money market securities. Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax. Municipal money market securities are high-quality, short-term debt securities that pay a fixed, variable, or floating interest rate, and include variable rate demand notes, commercial paper, and municipal notes. Municipal securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax. Potentially investing up to 20% of assets in securities subject to state and/or federal income tax. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.47%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 0.86%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-SelectComboPRO | Fidelity Investments Money Market Treasury Only Portfolio | Fidelity Investments Money Market Treasury Only Portfolio - Select Class                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.05%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.26%                          
Fee Waiver or Reimbursement [6] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.23%                          
Expense Example, with Redemption, 1 Year $ 24                          
Expense Example, with Redemption, 3 Years 80                          
Expense Example, with Redemption, 5 Years 142                          
Expense Example, with Redemption, 10 Years $ 327                          
Average Annual Return, Label [Optional Text]   Select Class                        
Average Annual Return, Percent   4.10%                     3.06% 2.00%
Annual Return [Percent]     4.10% 5.10% 4.83% 1.39% 0.01% 0.33% 1.99% 1.66% 0.66% 0.12%    
FIMMFunds-SelectComboPRO | Fidelity Investments Money Market Treasury Only Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Only Portfolio /Select                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Only Portfolio seeks as high a level of current income as is consistent with the security of principal and liquidity.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities. Normally investing in securities whose interest is exempt from state and local income taxes. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.87%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.31%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-SelectComboPRO | Fidelity Investments Money Market Treasury Portfolio | Fidelity Investments Money Market Treasury Portfolio - Select Class                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.05%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.26%                          
Fee Waiver or Reimbursement [7] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.23%                          
Expense Example, with Redemption, 1 Year $ 24                          
Expense Example, with Redemption, 3 Years 80                          
Expense Example, with Redemption, 5 Years 142                          
Expense Example, with Redemption, 10 Years $ 327                          
Average Annual Return, Label [Optional Text]   Select Class                        
Average Annual Return, Percent   4.13%                     3.12% 2.04%
Annual Return [Percent]     4.13% 5.10% 4.97% 1.48% 0.01% 0.32% 2.02% 1.66% 0.69% 0.15%    
FIMMFunds-SelectComboPRO | Fidelity Investments Money Market Treasury Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Portfolio /Select                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Treasury securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.87%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.31%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class1ComboPRO | Fidelity Investments Money Market Government Portfolio | Fidelity Investments Money Market Government Portfolio - Class I                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.21%                          
Fee Waiver or Reimbursement [8] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.18%                          
Expense Example, with Redemption, 1 Year $ 18                          
Expense Example, with Redemption, 3 Years 64                          
Expense Example, with Redemption, 5 Years 114                          
Expense Example, with Redemption, 10 Years $ 264                          
Average Annual Return, Label [Optional Text]   Class I                        
Average Annual Return, Percent   4.20%                     3.16% 2.09%
Annual Return [Percent]     4.20% 5.17% 5.03% 1.47% 0.01% 0.35% 2.09% 1.72% 0.75% 0.25%    
FIMMFunds-Class1ComboPRO | Fidelity Investments Money Market Government Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Government Portfolio /I                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Government Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities). Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities. U.S. Government securities are high-quality securities issued or guaranteed by the U.S. Treasury or by an agency or instrumentality of the U.S. Government. U.S. Government securities may be backed by the full faith and credit of the U.S. Treasury, the right to borrow from the U.S. Treasury, or the agency or instrumentality issuing or guaranteeing the security. Certain issuers of U.S. Government securities, including Fannie Mae, Freddie Mac, and the Federal Home Loan Banks, are sponsored or chartered by Congress but their securities are neither issued nor guaranteed by the U.S. Treasury. The issuer usually pays a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Government securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] www.fidelity.com or institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.88%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.33%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class1ComboPRO | Fidelity Investments Money Market Money Market Portfolio | Fidelity Investments Money Market - Money Market Portfolio - Class I                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.21%                          
Fee Waiver or Reimbursement [9] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.18%                          
Expense Example, with Redemption, 1 Year $ 18                          
Expense Example, with Redemption, 3 Years 64                          
Expense Example, with Redemption, 5 Years 114                          
Expense Example, with Redemption, 10 Years $ 264                          
Average Annual Return, Label [Optional Text]   Class I                        
Average Annual Return, Percent   4.25%                     3.24% 2.25%
Annual Return [Percent]     4.25% 5.23% 5.12% 1.68% 0.01% 0.52% 2.26% 2.00% 1.13% 0.48%    
FIMMFunds-Class1ComboPRO | Fidelity Investments Money Market Money Market Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Money Market Portfolio /I                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Money Market Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements. Investing more than 25% of total assets in the financial services industries. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] www.fidelity.com or institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.89%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.35%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Jun. 30, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class1ComboPRO | Fidelity Investments Money Market Tax-Exempt Portfolio | Fidelity Investments Money Market Tax Exempt Portfolio - Class I                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.08%                          
Expenses (as a percentage of Assets) 0.22%                          
Fee Waiver or Reimbursement [10] (0.04%)                          
Net Expenses (as a percentage of Assets) 0.18%                          
Expense Example, with Redemption, 1 Year $ 18                          
Expense Example, with Redemption, 3 Years 65                          
Expense Example, with Redemption, 5 Years 118                          
Expense Example, with Redemption, 10 Years $ 275                          
Average Annual Return, Label [Optional Text]   Class I                        
Average Annual Return, Percent   2.61%                     2.02% 1.43%
Annual Return [Percent]     2.61% 3.28% 3.20% 1.06% 0.01% 0.47% 1.36% 1.30% 0.71% 0.36%    
FIMMFunds-Class1ComboPRO | Fidelity Investments Money Market Tax-Exempt Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Tax-Exempt Portfolio /I                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Tax-Exempt Portfolio seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing in municipal money market securities. Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax. Municipal money market securities are high-quality, short-term debt securities that pay a fixed, variable, or floating interest rate, and include variable rate demand notes, commercial paper, and municipal notes. Municipal securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax. Potentially investing up to 20% of assets in securities subject to state and/or federal income tax. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] www.fidelity.com or institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.48%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 0.88%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class1ComboPRO | Fidelity Investments Money Market Treasury Only Portfolio | Fidelity Investments Money Market Treasury Only Portfolio - Class I                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.21%                          
Fee Waiver or Reimbursement [11] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.18%                          
Expense Example, with Redemption, 1 Year $ 18                          
Expense Example, with Redemption, 3 Years 64                          
Expense Example, with Redemption, 5 Years 114                          
Expense Example, with Redemption, 10 Years $ 264                          
Average Annual Return, Label [Optional Text]   Class I                        
Average Annual Return, Percent   4.15%                     3.10% 2.04%
Annual Return [Percent]     4.15% 5.15% 4.88% 1.43% 0.01% 0.36% 2.05% 1.71% 0.71% 0.17%    
FIMMFunds-Class1ComboPRO | Fidelity Investments Money Market Treasury Only Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Only Portfolio /I                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Only Portfolio seeks as high a level of current income as is consistent with the security of principal and liquidity.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities. Normally investing in securities whose interest is exempt from state and local income taxes. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] www.fidelity.com or institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.88%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.33%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class1ComboPRO | Fidelity Investments Money Market Treasury Portfolio | Fidelity Investments Money Market Treasury Portfolio - Class I                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.21%                          
Fee Waiver or Reimbursement [12] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.18%                          
Expense Example, with Redemption, 1 Year $ 18                          
Expense Example, with Redemption, 3 Years 64                          
Expense Example, with Redemption, 5 Years 114                          
Expense Example, with Redemption, 10 Years $ 264                          
Average Annual Return, Label [Optional Text]   Class I                        
Average Annual Return, Percent   4.18%                     3.16% 2.08%
Annual Return [Percent]     4.18% 5.15% 5.02% 1.52% 0.01% 0.35% 2.07% 1.72% 0.74% 0.20%    
FIMMFunds-Class1ComboPRO | Fidelity Investments Money Market Treasury Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Portfolio /I                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Treasury securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] www.fidelity.com or institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.88%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.32%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class2ComboPRO | Fidelity Investments Money Market Government Portfolio | Fidelity Investments Money Market Government Portfolio - Class II                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.15%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.36%                          
Fee Waiver or Reimbursement [13] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.33%                          
Expense Example, with Redemption, 1 Year $ 34                          
Expense Example, with Redemption, 3 Years 112                          
Expense Example, with Redemption, 5 Years 198                          
Expense Example, with Redemption, 10 Years $ 452                          
Average Annual Return, Label [Optional Text]   Class II                        
Average Annual Return, Percent   4.05%                     3.04% 1.96%
Annual Return [Percent]     4.05% 5.01% 4.87% 1.36% 0.01% 0.28% 1.93% 1.57% 0.60% 0.10%    
FIMMFunds-Class2ComboPRO | Fidelity Investments Money Market Government Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Government Portfolio /II                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Government Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities). Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities. U.S. Government securities are high-quality securities issued or guaranteed by the U.S. Treasury or by an agency or instrumentality of the U.S. Government. U.S. Government securities may be backed by the full faith and credit of the U.S. Treasury, the right to borrow from the U.S. Treasury, or the agency or instrumentality issuing or guaranteeing the security. Certain issuers of U.S. Government securities, including Fannie Mae, Freddie Mac, and the Federal Home Loan Banks, are sponsored or chartered by Congress but their securities are neither issued nor guaranteed by the U.S. Treasury. The issuer usually pays a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Government securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.84%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.29%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class2ComboPRO | Fidelity Investments Money Market Money Market Portfolio | Fidelity Investments Money Market - Money Market Portfolio - Class II                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.15%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.36%                          
Fee Waiver or Reimbursement [14] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.33%                          
Expense Example, with Redemption, 1 Year $ 34                          
Expense Example, with Redemption, 3 Years 112                          
Expense Example, with Redemption, 5 Years 198                          
Expense Example, with Redemption, 10 Years $ 452                          
Average Annual Return, Label [Optional Text]   Class II                        
Average Annual Return, Percent   4.09%                     3.12% 2.12%
Annual Return [Percent]     4.09% 5.07% 4.97% 1.56% 0.01% 0.42% 2.10% 1.85% 0.98% 0.33%    
FIMMFunds-Class2ComboPRO | Fidelity Investments Money Market Money Market Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Money Market Portfolio /II                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Money Market Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements. Investing more than 25% of total assets in the financial services industries. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.86%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.31%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class2ComboPRO | Fidelity Investments Money Market Tax-Exempt Portfolio | Fidelity Investments Money Market Tax Exempt Portfolio - Class II                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.15%                          
Other Expenses (as a percentage of Assets): 0.08%                          
Expenses (as a percentage of Assets) 0.37%                          
Fee Waiver or Reimbursement [15] (0.04%)                          
Net Expenses (as a percentage of Assets) 0.33%                          
Expense Example, with Redemption, 1 Year $ 34                          
Expense Example, with Redemption, 3 Years 113                          
Expense Example, with Redemption, 5 Years 202                          
Expense Example, with Redemption, 10 Years $ 463                          
Average Annual Return, Label [Optional Text]   Class II                        
Average Annual Return, Percent   2.46%                     1.91% 1.31%
Annual Return [Percent]     2.46% 3.12% 3.05% 0.93% 0.01% 0.38% 1.21% 1.15% 0.56% 0.25%    
FIMMFunds-Class2ComboPRO | Fidelity Investments Money Market Tax-Exempt Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Tax-Exempt Portfolio /II                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Tax-Exempt Portfolio seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing in municipal money market securities. Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax. Municipal money market securities are high-quality, short-term debt securities that pay a fixed, variable, or floating interest rate, and include variable rate demand notes, commercial paper, and municipal notes. Municipal securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax. Potentially investing up to 20% of assets in securities subject to state and/or federal income tax. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.45%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 0.84%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class2ComboPRO | Fidelity Investments Money Market Treasury Only Portfolio | Fidelity Investments Money Market Treasury Only Portfolio - Class II                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.15%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.36%                          
Fee Waiver or Reimbursement [16] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.33%                          
Expense Example, with Redemption, 1 Year $ 34                          
Expense Example, with Redemption, 3 Years 112                          
Expense Example, with Redemption, 5 Years 198                          
Expense Example, with Redemption, 10 Years $ 452                          
Average Annual Return, Label [Optional Text]   Class II                        
Average Annual Return, Percent   3.99%                     2.99% 1.92%
Annual Return [Percent]     3.99% 4.99% 4.72% 1.31% 0.01% 0.29% 1.89% 1.56% 0.56% 0.03%    
FIMMFunds-Class2ComboPRO | Fidelity Investments Money Market Treasury Only Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Only Portfolio /II                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Only Portfolio seeks as high a level of current income as is consistent with the security of principal and liquidity.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities. Normally investing in securities whose interest is exempt from state and local income taxes. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.84%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.29%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class2ComboPRO | Fidelity Investments Money Market Treasury Portfolio | Fidelity Investments Money Market Treasury Portfolio - Class II                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.15%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.36%                          
Fee Waiver or Reimbursement [17] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.33%                          
Expense Example, with Redemption, 1 Year $ 34                          
Expense Example, with Redemption, 3 Years 112                          
Expense Example, with Redemption, 5 Years 198                          
Expense Example, with Redemption, 10 Years $ 452                          
Average Annual Return, Label [Optional Text]   Class II                        
Average Annual Return, Percent   4.02%                     3.04% 1.95%
Annual Return [Percent]     4.02% 4.99% 4.87% 1.40% 0.01% 0.28% 1.92% 1.56% 0.59% 0.06%    
FIMMFunds-Class2ComboPRO | Fidelity Investments Money Market Treasury Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Portfolio /II                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Treasury securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.85%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.29%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class3ComboPRO | Fidelity Investments Money Market Government Portfolio | Fidelity Investments Money Market Government Portfolio - Class III                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.25%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.46%                          
Fee Waiver or Reimbursement [18] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.43%                          
Expense Example, with Redemption, 1 Year $ 44                          
Expense Example, with Redemption, 3 Years 144                          
Expense Example, with Redemption, 5 Years 254                          
Expense Example, with Redemption, 10 Years $ 575                          
Average Annual Return, Label [Optional Text]   Class III                        
Average Annual Return, Percent   3.94%                     2.96% 1.88%
Annual Return [Percent]     3.94% 4.91% 4.77% 1.29% 0.01% 0.26% 1.83% 1.47% 0.50% 0.02%    
FIMMFunds-Class3ComboPRO | Fidelity Investments Money Market Government Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Government Portfolio /III                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Government Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities). Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities. U.S. Government securities are high-quality securities issued or guaranteed by the U.S. Treasury or by an agency or instrumentality of the U.S. Government. U.S. Government securities may be backed by the full faith and credit of the U.S. Treasury, the right to borrow from the U.S. Treasury, or the agency or instrumentality issuing or guaranteeing the security. Certain issuers of U.S. Government securities, including Fannie Mae, Freddie Mac, and the Federal Home Loan Banks, are sponsored or chartered by Congress but their securities are neither issued nor guaranteed by the U.S. Treasury. The issuer usually pays a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Government securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.82%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.26%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class3ComboPRO | Fidelity Investments Money Market Money Market Portfolio | Fidelity Investments Money Market - Money Market Portfolio - Class III                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.25%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.46%                          
Fee Waiver or Reimbursement [19] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.43%                          
Expense Example, with Redemption, 1 Year $ 44                          
Expense Example, with Redemption, 3 Years 144                          
Expense Example, with Redemption, 5 Years 254                          
Expense Example, with Redemption, 10 Years $ 575                          
Average Annual Return, Label [Optional Text]   Class III                        
Average Annual Return, Percent   3.99%                     3.04% 2.04%
Annual Return [Percent]     3.99% 4.97% 4.86% 1.48% 0.01% 0.38% 2.00% 1.75% 0.88% 0.23%    
FIMMFunds-Class3ComboPRO | Fidelity Investments Money Market Money Market Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Money Market Portfolio /III                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Money Market Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements. Investing more than 25% of total assets in the financial services industries. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.83%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.28%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class3ComboPRO | Fidelity Investments Money Market Tax-Exempt Portfolio | Fidelity Investments Money Market Tax Exempt Portfolio - Class III                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.25%                          
Other Expenses (as a percentage of Assets): 0.08%                          
Expenses (as a percentage of Assets) 0.47%                          
Fee Waiver or Reimbursement [20] (0.04%)                          
Net Expenses (as a percentage of Assets) 0.43%                          
Expense Example, with Redemption, 1 Year $ 44                          
Expense Example, with Redemption, 3 Years 145                          
Expense Example, with Redemption, 5 Years 258                          
Expense Example, with Redemption, 10 Years $ 586                          
Average Annual Return, Label [Optional Text]   Class III                        
Average Annual Return, Percent   2.36%                     1.83% 1.23%
Annual Return [Percent]     2.36% 3.02% 2.95% 0.85% 0.01% 0.34% 1.11% 1.05% 0.46% 0.18%    
FIMMFunds-Class3ComboPRO | Fidelity Investments Money Market Tax-Exempt Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Tax-Exempt Portfolio /III                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Tax-Exempt Portfolio seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing in municipal money market securities. Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax. Municipal money market securities are high-quality, short-term debt securities that pay a fixed, variable, or floating interest rate, and include variable rate demand notes, commercial paper, and municipal notes. Municipal securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax. Potentially investing up to 20% of assets in securities subject to state and/or federal income tax. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.42%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 0.81%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class3ComboPRO | Fidelity Investments Money Market Treasury Only Portfolio | Fidelity Investments Money Market Treasury Only Portfolio - Class III                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.25%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.46%                          
Fee Waiver or Reimbursement [21] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.43%                          
Expense Example, with Redemption, 1 Year $ 44                          
Expense Example, with Redemption, 3 Years 144                          
Expense Example, with Redemption, 5 Years 254                          
Expense Example, with Redemption, 10 Years $ 575                          
Average Annual Return, Label [Optional Text]   Class III                        
Average Annual Return, Percent   3.89%                     2.91% 1.85%
Annual Return [Percent]     3.89% 4.89% 4.62% 1.24% 0.01% 0.26% 1.79% 1.46% 0.46% 0.01%    
FIMMFunds-Class3ComboPRO | Fidelity Investments Money Market Treasury Only Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Only Portfolio /III                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Only Portfolio seeks as high a level of current income as is consistent with the security of principal and liquidity.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities. Normally investing in securities whose interest is exempt from state and local income taxes. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.82%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.26%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-Class3ComboPRO | Fidelity Investments Money Market Treasury Portfolio | Fidelity Investments Money Market Treasury Portfolio - Class III                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.25%                          
Other Expenses (as a percentage of Assets): 0.07%                          
Expenses (as a percentage of Assets) 0.46%                          
Fee Waiver or Reimbursement [22] (0.03%)                          
Net Expenses (as a percentage of Assets) 0.43%                          
Expense Example, with Redemption, 1 Year $ 44                          
Expense Example, with Redemption, 3 Years 144                          
Expense Example, with Redemption, 5 Years 254                          
Expense Example, with Redemption, 10 Years $ 575                          
Average Annual Return, Label [Optional Text]   Class III                        
Average Annual Return, Percent   3.92%                     2.96% 1.88%
Annual Return [Percent]     3.92% 4.89% 4.76% 1.33% 0.01% 0.25% 1.82% 1.46% 0.49% 0.01%    
FIMMFunds-Class3ComboPRO | Fidelity Investments Money Market Treasury Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Portfolio /III                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Treasury securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.82%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.26%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-InstitutionalComboPRO | Fidelity Investments Money Market Government Portfolio | Fidelity Investments Money Market Government Portfolio - Institutional Class                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.04%                          
Expenses (as a percentage of Assets) 0.18%                          
Fee Waiver or Reimbursement [23] (0.04%)                          
Net Expenses (as a percentage of Assets) 0.14%                          
Expense Example, with Redemption, 1 Year $ 14                          
Expense Example, with Redemption, 3 Years 53                          
Expense Example, with Redemption, 5 Years 96                          
Expense Example, with Redemption, 10 Years $ 225                          
Average Annual Return, Label [Optional Text]   Institutional Class                        
Average Annual Return, Percent   4.24%                     3.19% 2.12%
Annual Return [Percent]     4.24% 5.21% 5.07% 1.51% 0.01% 0.38% 2.13% 1.76% 0.79% 0.29%    
FIMMFunds-InstitutionalComboPRO | Fidelity Investments Money Market Government Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Government Portfolio /Institutional                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Government Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities). Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities. U.S. Government securities are high-quality securities issued or guaranteed by the U.S. Treasury or by an agency or instrumentality of the U.S. Government. U.S. Government securities may be backed by the full faith and credit of the U.S. Treasury, the right to borrow from the U.S. Treasury, or the agency or instrumentality issuing or guaranteeing the security. Certain issuers of U.S. Government securities, including Fannie Mae, Freddie Mac, and the Federal Home Loan Banks, are sponsored or chartered by Congress but their securities are neither issued nor guaranteed by the U.S. Treasury. The issuer usually pays a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Government securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] www.fidelity.com or institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.89%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.34%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-InstitutionalComboPRO | Fidelity Investments Money Market Money Market Portfolio | Fidelity Investments Money Market - Money Market Portfolio - Institutional Class                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.04%                          
Expenses (as a percentage of Assets) 0.18%                          
Fee Waiver or Reimbursement [24] (0.04%)                          
Net Expenses (as a percentage of Assets) 0.14%                          
Expense Example, with Redemption, 1 Year $ 14                          
Expense Example, with Redemption, 3 Years 53                          
Expense Example, with Redemption, 5 Years 96                          
Expense Example, with Redemption, 10 Years $ 225                          
Average Annual Return, Label [Optional Text]   Institutional Class                        
Average Annual Return, Percent   4.29%                     3.28% 2.29%
Annual Return [Percent]     4.29% 5.27% 5.17% 1.72% 0.03% 0.56% 2.30% 2.04% 1.17% 0.52%    
FIMMFunds-InstitutionalComboPRO | Fidelity Investments Money Market Money Market Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Money Market Portfolio /Institutional                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Money Market Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements. Investing more than 25% of total assets in the financial services industries. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] www.fidelity.com or institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.90%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.36%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Sep. 30, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-InstitutionalComboPRO | Fidelity Investments Money Market Treasury Only Portfolio | Fidelity Investments Money Market Treasury Only Portfolio - Institutional Class                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.04%                          
Expenses (as a percentage of Assets) 0.18%                          
Fee Waiver or Reimbursement [25] (0.04%)                          
Net Expenses (as a percentage of Assets) 0.14%                          
Expense Example, with Redemption, 1 Year $ 14                          
Expense Example, with Redemption, 3 Years 53                          
Expense Example, with Redemption, 5 Years 96                          
Expense Example, with Redemption, 10 Years $ 225                          
Average Annual Return, Label [Optional Text]   Institutional Class                        
Average Annual Return, Percent   4.19%                     3.14% 2.08%
Annual Return [Percent]     4.19% 5.19% 4.92% 1.47% 0.01% 0.38% 2.09% 1.75% 0.75% 0.21%    
FIMMFunds-InstitutionalComboPRO | Fidelity Investments Money Market Treasury Only Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Only Portfolio /Institutional                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Only Portfolio seeks as high a level of current income as is consistent with the security of principal and liquidity.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities. Normally investing in securities whose interest is exempt from state and local income taxes. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] www.fidelity.com or institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.89%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.34%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
FIMMFunds-InstitutionalComboPRO | Fidelity Investments Money Market Treasury Portfolio | Fidelity Investments Money Market Treasury Portfolio - Institutional Class                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.14%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.04%                          
Expenses (as a percentage of Assets) 0.18%                          
Fee Waiver or Reimbursement [26] (0.04%)                          
Net Expenses (as a percentage of Assets) 0.14%                          
Expense Example, with Redemption, 1 Year $ 14                          
Expense Example, with Redemption, 3 Years 53                          
Expense Example, with Redemption, 5 Years 96                          
Expense Example, with Redemption, 10 Years $ 225                          
Average Annual Return, Label [Optional Text]   Institutional Class                        
Average Annual Return, Percent   4.22%                     3.19% 2.11%
Annual Return [Percent]     4.22% 5.19% 5.06% 1.55% 0.01% 0.38% 2.11% 1.76% 0.78% 0.24%    
FIMMFunds-InstitutionalComboPRO | Fidelity Investments Money Market Treasury Portfolio                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund /Class: Treasury Portfolio /Institutional                            
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Treasury Portfolio seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.                          
Expense Heading [Optional Text] Fee Table                          
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                          
Shareholder Fees Caption [Optional Text] Shareholder fees                          
Shareholder Fee, Other $ 0                          
Operating Expenses Caption [Optional Text] Annual Operating Expenses                          
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                          
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                          
Strategy [Heading] Principal Investment Strategies                          
Strategy Narrative [Text Block] Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities. Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. A repurchase agreement is an agreement to buy a security at one price and a simultaneous agreement to sell it back at an agreed-upon price. A repurchase agreement entered into by the fund may be collateralized by U.S. Treasury securities or cash.                          
Bar Chart and Performance Table [Heading] Performance                          
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.                          
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                          
Performance Availability Website Address [Text] www.fidelity.com or institutional.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                          
Bar Chart [Heading] Year-by-Year Returns                          
Year to Date Return, Label [Optional Text]    Year-to-Date Return                          
Bar Chart, Year to Date Return, Date Mar. 31, 2026                          
Bar Chart, Year to Date Return 0.89%                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 1.33%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2021                          
Lowest Quarterly Return none                          
Performance Table Heading Average Annual Returns                          
Document Type 485BPOS                          
Registrant Name Fidelity Colchester Street Trust                          
[1]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class IV of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.68% (the Expense Cap). If at any time during the current fiscal year expenses for Class IV of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[2]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class IV of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.68% (the Expense Cap). If at any time during the current fiscal year expenses for Class IV of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[3]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[4]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[5]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[6]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[7]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[8]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[9]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[10]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[11]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[12]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[13]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[14]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[15]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[16]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[17]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[18]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[19]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[20]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[21]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[22]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[23]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[24]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[25]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[26]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2027 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.