v3.26.1
Shareholder Report
 12 Months Ended 92 Months Ended
Apr. 30, 2026
USD ($)
shares
Apr. 30, 2026
USD ($)
shares
Shareholder Report [Line Items]    
Document Type N-CSR  
Amendment Flag false  
Registrant Name Fidelity Hereford Street Trust  
Entity Central Index Key 0000917286  
Entity Investment Company Type N-1A  
Document Period End Date Apr. 30, 2026  
Fidelity Treasury Only Money Market Fund    
Shareholder Report [Line Items]    
Fund Name Fidelity® Treasury Only Money Market Fund  
Class Name Fidelity® Treasury Only Money Market Fund  
Trading Symbol FDLXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Treasury Only Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Material Fund Change Notice [Text Block] This report describes changes to the Fund that occurred during the reporting period.  
Additional Information Phone Number 1-800-544-8544  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Treasury Only Money Market Fund 
$ 43 
0.42%
  
 
Expenses Paid, Amount $ 43  
Expense Ratio, Percent 0.42%  
Net Assets $ 16,940,567,988 $ 16,940,567,988
Holdings Count | shares 57 57
Advisory Fees Paid, Amount $ 61,283,591  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$16,940,567,988
  
Number of Holdings
57
  
Total Advisory Fee
$61,283,591
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 19.9   8-30 17.9   31-60 24.1   61-90 21.0   91-180 16.4   >180 1.6             U.S. Treasury Obligations 100.9 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 100.9        Net Other Assets (Liabilities) - (0.9)%    
Material Fund Change [Text Block]
How has the Fund changed?
 
This is a summary of certain changes to the Fund since May 1, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by June 27, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-800-544-8544  or by sending an e-mail to fidfunddocuments@fidelity.com.
 
The fund modified its principal investment strategies during the reporting period.
The fund modified its principal investment risks during the reporting period.

 
Material Fund Change Strategies [Text Block] The fund modified its principal investment strategies during the reporting period.
 
Material Fund Change Risks Change [Text Block] The fund modified its principal investment risks during the reporting period.

 
Summary of Change Legend [Text Block] This is a summary of certain changes to the Fund since May 1, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by June 27, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-800-544-8544  or by sending an e-mail to fidfunddocuments@fidelity.com.  
Updated Prospectus Phone Number <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">1-800-544-8544</span>  
Updated Prospectus Email Address <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">fidfunddocuments@fidelity.com</span>  
Updated Prospectus Web Address <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:bold;">fundresearch.fidelity.com/prospectus/sec</span>  
Fidelity Series Treasury Bill Index Fund    
Shareholder Report [Line Items]    
Fund Name Fidelity® Series Treasury Bill Index Fund  
Class Name Fidelity® Series Treasury Bill Index Fund  
Trading Symbol FHQFX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Series Treasury Bill Index Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544.  
Additional Information Phone Number 1-800-544-8544  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Series Treasury Bill Index Fund 
$ 0 A
0.00%B
  
A Amount represents less than $.50
B Amount represents less than 0.005%
 
  
Expenses Paid, Amount $ 0  
Expense Ratio, Percent 0.00%  
Factors Affecting Performance [Text Block] What affected the Fund's performance this period?
 
•U.S. taxable investment-grade bonds achieved a solid gain for the 12 months ending April 30, 2026, navigating uncertainty related to U.S. trade policy and geopolitical events but buoyed by attractive starting yields, a resilient economy and three policy interest-rate cuts by the Federal Reserve in the second half of 2025. 
•We construct the portfolio using optimization. This approach minimizes the differences between risk exposures of the fund relative to the index. We attempt to hold all positions held by the index in the same relative proportions.
•Exposures include duration, key-rate durations and other factors.
  
Performance Past Does Not Indicate Future [Text] <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:italic;font-weight:normal;">The Fund's past performance is not a good predictor of the Fund's future performance.</span>  
Line Graph [Table Text Block] How did the Fund perform over the life of Fund?    CUMULATIVE PERFORMANCE August 17, 2018 through April 30, 2026. Initial investment of $10,000. Fidelity® Series Treasury Bill Index Fund $10,000 $10,162 $10,400 $10,413 $10,418 $10,707 $11,280 $11,854 $12,331 Bloomberg U.S. 3-6 Month Treasury Bill Index $10,000 $10,167 $10,407 $10,421 $10,422 $10,715 $11,290 $11,857 $12,342 Bloomberg U.S. Aggregate Bond Index $10,000 $10,413 $11,542 $11,511 $10,531 $10,486 $10,332 $11,161 $11,613   2018 2019 2020 2021 2022 2023 2024 2025 2026    
Average Annual Return [Table Text Block] AVERAGE ANNUAL TOTAL RETURNS:   1 Year 5 Year Life of Fund A Fidelity® Series Treasury Bill Index Fund 4.02% 3.44% 2.76% Bloomberg U.S. 3-6 Month Treasury Bill Index 4.09% 3.44% 2.77% Bloomberg U.S. Aggregate Bond Index 4.06% 0.18% 1.96% A   From August 17, 2018   Visit www.fidelity.com for more recent performance information.    The Fund's past performance is not a good predictor of the Fund's future performance.  The graph and table do not reflect the deduction of taxes that a shareholder would pay on fund distributions or redemption of fund shares.  
Performance Inception Date   Aug. 17, 2018
No Deduction of Taxes [Text Block] The graph and table do not reflect the deduction of taxes that a shareholder would pay on fund distributions or redemption of fund shares.  
Updated Performance Information Location [Text Block] Visit www.fidelity.com for more recent performance information.  
Net Assets $ 2,030,384,119 $ 2,030,384,119
Holdings Count | shares 23 23
Advisory Fees Paid, Amount $ 0  
Investment Company Portfolio Turnover 0.00%  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$2,030,384,119
  
Number of Holdings
23
  
Total Advisory Fee
$0
  
Portfolio Turnover
0%
  
 
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) Zero coupon bonds 99.2         U.S. Treasury Obligations 99.2 Short-Term Investments and Net Other Assets (Liabilities) 0.8 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 99.2           Short-Term Investments and Net Other Assets (Liabilities) - 0.8    
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Bill 99.2     99.2    
Fidelity Money Market Fund    
Shareholder Report [Line Items]    
Fund Name Fidelity® Money Market Fund  
Class Name Fidelity® Money Market Fund  
Trading Symbol SPRXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Additional Information Phone Number 1-800-544-8544  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Money Market Fund 
$ 43 
0.42%
  
 
Expenses Paid, Amount $ 43  
Expense Ratio, Percent 0.42%  
Net Assets $ 134,691,553,806 $ 134,691,553,806
Holdings Count | shares 266 266
Advisory Fees Paid, Amount $ 337,385,430  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$134,691,553,806
  
Number of Holdings
266
  
Total Advisory Fee
$337,385,430
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 53.8   8-30 7.9   31-60 12.1   61-90 15.0   91-180 12.9   >180 0.6             Repurchase Agreements 50.9 U.S. Treasury Obligations 32.6 Commercial Paper 8.4 Time Deposits 5.6 Certificates of Deposit 4.8 ASSET ALLOCATION (% of Fund's net assets)     Repurchase Agreements - 50.9               U.S. Treasury Obligations - 32.6           Commercial Paper - 8.4                     Time Deposits - 5.6                        Certificates of Deposit - 4.8            Net Other Assets (Liabilities) - (2.3)%    
Fidelity Money Market Fund - Premium Class    
Shareholder Report [Line Items]    
Fund Name Fidelity® Money Market Fund  
Class Name Fidelity® Money Market Fund Premium Class  
Trading Symbol FZDXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Additional Information Phone Number 1-800-544-8544  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Premium Class 
$ 31 
0.30%
  
 
Expenses Paid, Amount $ 31  
Expense Ratio, Percent 0.30%  
Net Assets $ 134,691,553,806 $ 134,691,553,806
Holdings Count | shares 266 266
Advisory Fees Paid, Amount $ 337,385,430  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$134,691,553,806
  
Number of Holdings
266
  
Total Advisory Fee
$337,385,430
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 53.8   8-30 7.9   31-60 12.1   61-90 15.0   91-180 12.9   >180 0.6             Repurchase Agreements 50.9 U.S. Treasury Obligations 32.6 Commercial Paper 8.4 Time Deposits 5.6 Certificates of Deposit 4.8 ASSET ALLOCATION (% of Fund's net assets)     Repurchase Agreements - 50.9               U.S. Treasury Obligations - 32.6           Commercial Paper - 8.4                     Time Deposits - 5.6                        Certificates of Deposit - 4.8            Net Other Assets (Liabilities) - (2.3)%    
Fidelity Government Money Market Fund    
Shareholder Report [Line Items]    
Fund Name Fidelity® Government Money Market Fund  
Class Name Fidelity® Government Money Market Fund  
Trading Symbol SPAXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Additional Information Phone Number 1-800-544-8544  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Government Money Market Fund 
$ 43 
0.42%
  
 
Expenses Paid, Amount $ 43  
Expense Ratio, Percent 0.42%  
Net Assets $ 452,711,941,602 $ 452,711,941,602
Holdings Count | shares 744 744
Advisory Fees Paid, Amount $ 1,063,880,910  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$452,711,941,602
  
Number of Holdings
744
  
Total Advisory Fee
$1,063,880,910
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 62.9   8-30 7.9   31-60 5.3   61-90 7.6   91-180 10.4   >180 7.1             Repurchase Agreements 41.0 U.S. Government Agency - Debt 31.6 U.S. Treasury Obligations 28.6 ASSET ALLOCATION (% of Fund's net assets)     Repurchase Agreements - 41.0               U.S. Government Agency - Debt - 31.6       U.S. Treasury Obligations - 28.6         Net Other Assets (Liabilities) - (1.2)%    
Fidelity Government Money Market Fund - Premium Class    
Shareholder Report [Line Items]    
Fund Name Fidelity® Government Money Market Fund  
Class Name Fidelity® Government Money Market Fund Premium Class  
Trading Symbol FZCXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Additional Information Phone Number 1-800-544-8544  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Premium Class 
$ 33 
0.32%
  
 
Expenses Paid, Amount $ 33  
Expense Ratio, Percent 0.32%  
Net Assets $ 452,711,941,602 $ 452,711,941,602
Holdings Count | shares 744 744
Advisory Fees Paid, Amount $ 1,063,880,910  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$452,711,941,602
  
Number of Holdings
744
  
Total Advisory Fee
$1,063,880,910
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 62.9   8-30 7.9   31-60 5.3   61-90 7.6   91-180 10.4   >180 7.1             Repurchase Agreements 41.0 U.S. Government Agency - Debt 31.6 U.S. Treasury Obligations 28.6 ASSET ALLOCATION (% of Fund's net assets)     Repurchase Agreements - 41.0               U.S. Government Agency - Debt - 31.6       U.S. Treasury Obligations - 28.6         Net Other Assets (Liabilities) - (1.2)%    
Fidelity Government Money Market Fund - Daily Money Class    
Shareholder Report [Line Items]    
Fund Name Fidelity® Government Money Market Fund  
Class Name Fidelity® Government Money Market Fund Daily Money Class  
Trading Symbol FZBXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-877-208-0098 or by sending an e-mail to funddocuments@fmr.com.  
Additional Information Phone Number 1-877-208-0098  
Additional Information Email funddocuments@fmr.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Daily Money Class 
$ 71 
0.70%
  
 
Expenses Paid, Amount $ 71  
Expense Ratio, Percent 0.70%  
Net Assets $ 452,711,941,602 $ 452,711,941,602
Holdings Count | shares 744 744
Advisory Fees Paid, Amount $ 1,063,880,910  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$452,711,941,602
  
Number of Holdings
744
  
Total Advisory Fee
$1,063,880,910
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 62.9   8-30 7.9   31-60 5.3   61-90 7.6   91-180 10.4   >180 7.1             Repurchase Agreements 41.0 U.S. Government Agency - Debt 31.6 U.S. Treasury Obligations 28.6 ASSET ALLOCATION (% of Fund's net assets)     Repurchase Agreements - 41.0               U.S. Government Agency - Debt - 31.6       U.S. Treasury Obligations - 28.6         Net Other Assets (Liabilities) - (1.2)%    
Fidelity Government Money Market Fund - Class S    
Shareholder Report [Line Items]    
Fund Name Fidelity® Government Money Market Fund  
Class Name Fidelity® Government Money Market Fund Class S  
Trading Symbol FZSXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Additional Information Phone Number 1-800-544-8544  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Class S 
$ 43 
0.42%
  
 
Expenses Paid, Amount $ 43  
Expense Ratio, Percent 0.42%  
Net Assets $ 452,711,941,602 $ 452,711,941,602
Holdings Count | shares 744 744
Advisory Fees Paid, Amount $ 1,063,880,910  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$452,711,941,602
  
Number of Holdings
744
  
Total Advisory Fee
$1,063,880,910
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 62.9   8-30 7.9   31-60 5.3   61-90 7.6   91-180 10.4   >180 7.1             Repurchase Agreements 41.0 U.S. Government Agency - Debt 31.6 U.S. Treasury Obligations 28.6 ASSET ALLOCATION (% of Fund's net assets)     Repurchase Agreements - 41.0               U.S. Government Agency - Debt - 31.6       U.S. Treasury Obligations - 28.6         Net Other Assets (Liabilities) - (1.2)%    
Fidelity Government Money Market Fund - Class K6    
Shareholder Report [Line Items]    
Fund Name Fidelity® Government Money Market Fund  
Class Name Fidelity® Government Money Market Fund Class K6  
Trading Symbol FNBXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-835-5092 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Additional Information Phone Number 1-800-835-5092  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Class K6 
$ 26 
0.25%
  
 
Expenses Paid, Amount $ 26  
Expense Ratio, Percent 0.25%  
Net Assets $ 452,711,941,602 $ 452,711,941,602
Holdings Count | shares 744 744
Advisory Fees Paid, Amount $ 1,063,880,910  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$452,711,941,602
  
Number of Holdings
744
  
Total Advisory Fee
$1,063,880,910
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 62.9   8-30 7.9   31-60 5.3   61-90 7.6   91-180 10.4   >180 7.1             Repurchase Agreements 41.0 U.S. Government Agency - Debt 31.6 U.S. Treasury Obligations 28.6 ASSET ALLOCATION (% of Fund's net assets)     Repurchase Agreements - 41.0               U.S. Government Agency - Debt - 31.6       U.S. Treasury Obligations - 28.6         Net Other Assets (Liabilities) - (1.2)%    
Fidelity Government Money Market Fund - Capital Reserves Class    
Shareholder Report [Line Items]    
Fund Name Fidelity® Government Money Market Fund  
Class Name Fidelity® Government Money Market Fund Capital Reserves Class  
Trading Symbol FZAXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-877-208-0098 or by sending an e-mail to funddocuments@fmr.com.  
Additional Information Phone Number 1-877-208-0098  
Additional Information Email funddocuments@fmr.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Capital Reserves Class 
$ 97 
0.95%
  
 
Expenses Paid, Amount $ 97  
Expense Ratio, Percent 0.95%  
Net Assets $ 452,711,941,602 $ 452,711,941,602
Holdings Count | shares 744 744
Advisory Fees Paid, Amount $ 1,063,880,910  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$452,711,941,602
  
Number of Holdings
744
  
Total Advisory Fee
$1,063,880,910
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 62.9   8-30 7.9   31-60 5.3   61-90 7.6   91-180 10.4   >180 7.1             Repurchase Agreements 41.0 U.S. Government Agency - Debt 31.6 U.S. Treasury Obligations 28.6 ASSET ALLOCATION (% of Fund's net assets)     Repurchase Agreements - 41.0               U.S. Government Agency - Debt - 31.6       U.S. Treasury Obligations - 28.6         Net Other Assets (Liabilities) - (1.2)%    
Fidelity Government Money Market Fund - Class M    
Shareholder Report [Line Items]    
Fund Name Fidelity® Government Money Market Fund  
Class Name Fidelity® Government Money Market Fund Advisor M Class  
Trading Symbol FZGXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-877-208-0098 or by sending an e-mail to funddocuments@fmr.com.  
Additional Information Phone Number 1-877-208-0098  
Additional Information Email funddocuments@fmr.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Advisor M Class 
$ 71 
0.70%
  
 
Expenses Paid, Amount $ 71  
Expense Ratio, Percent 0.70%  
Net Assets $ 452,711,941,602 $ 452,711,941,602
Holdings Count | shares 744 744
Advisory Fees Paid, Amount $ 1,063,880,910  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$452,711,941,602
  
Number of Holdings
744
  
Total Advisory Fee
$1,063,880,910
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 62.9   8-30 7.9   31-60 5.3   61-90 7.6   91-180 10.4   >180 7.1             Repurchase Agreements 41.0 U.S. Government Agency - Debt 31.6 U.S. Treasury Obligations 28.6 ASSET ALLOCATION (% of Fund's net assets)     Repurchase Agreements - 41.0               U.S. Government Agency - Debt - 31.6       U.S. Treasury Obligations - 28.6         Net Other Assets (Liabilities) - (1.2)%    
Fidelity Flex Government Money Market Fund    
Shareholder Report [Line Items]    
Fund Name Fidelity Flex® Government Money Market Fund  
Class Name Fidelity Flex® Government Money Market Fund  
Trading Symbol FLGXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity Flex® Government Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-3455 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Material Fund Change Notice [Text Block] This report describes changes to the Fund that occurred during the reporting period.  
Additional Information Phone Number 1-800-544-3455  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity Flex® Government Money Market Fund 
$ 0 A
0.00%B
  
A Amount represents less than $.50
B Amount represents less than 0.005%
 
  
Expenses Paid, Amount $ 0  
Expense Ratio, Percent 0.00%  
Net Assets $ 221,277,787 $ 221,277,787
Holdings Count | shares 173 173
Advisory Fees Paid, Amount $ 0  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$221,277,787
  
Number of Holdings
173
  
Total Advisory Fee
$0
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 59.4   8-30 8.2   31-60 5.5   61-90 7.1   91-180 15.2   >180 4.1             Repurchase Agreements 38.3 U.S. Government Agency - Debt 33.8 U.S. Treasury Obligations 27.4 Net Other Assets (Liabilities) 0.5 ASSET ALLOCATION (% of Fund's net assets)     Repurchase Agreements - 38.3               U.S. Government Agency - Debt - 33.8       U.S. Treasury Obligations - 27.4           Net Other Assets (Liabilities) - 0.5        
Material Fund Change [Text Block]
How has the Fund changed?
 
This is a summary of certain changes to the Fund since May 1, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by June 27, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-800-544-3455  or by sending an e-mail to fidfunddocuments@fidelity.com.
 
The fund's contractual proxy and shareholder meeting expense cap expired during the reporting period.
 
Material Fund Change Expenses [Text Block] The fund's contractual proxy and shareholder meeting expense cap expired during the reporting period.
 
Summary of Change Legend [Text Block] This is a summary of certain changes to the Fund since May 1, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by June 27, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-800-544-3455  or by sending an e-mail to fidfunddocuments@fidelity.com.  
Updated Prospectus Phone Number <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">1-800-544-3455</span>  
Updated Prospectus Email Address <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">fidfunddocuments@fidelity.com</span>  
Updated Prospectus Web Address <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:bold;">fundresearch.fidelity.com/prospectus/sec</span>  
Fidelity Treasury Digital Fund - Liquidity    
Shareholder Report [Line Items]    
Fund Name Fidelity® Treasury Digital Fund  
Class Name Fidelity® Treasury Digital Fund Liquidity  
Trading Symbol FYHXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Treasury Digital Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-877-297-2952 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Material Fund Change Notice [Text Block] This report describes changes to the Fund that occurred during the reporting period.  
Additional Information Phone Number 1-877-297-2952  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Liquidity 
$ 19 
0.18%
  
 
Expenses Paid, Amount $ 19  
Expense Ratio, Percent 0.18%  
Net Assets $ 171,679,853 $ 171,679,853
Holdings Count | shares 17 17
Advisory Fees Paid, Amount $ 621,595  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$171,679,853
  
Number of Holdings
17
  
Total Advisory Fee
$621,595
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 10.5   8-30 24.2   31-60 38.5   61-90 31.5             U.S. Treasury Obligations 104.7 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 104.7        Net Other Assets (Liabilities) - (4.7)%    
Material Fund Change [Text Block]
How has the Fund changed?
 
This is a summary of certain changes to the Fund since May 1, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by June 27, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-877-297-2952  or by sending an e-mail to fidfunddocuments@fidelity.com.
 
The fund modified its principal investment strategies during the reporting period.
The fund modified its principal investment risks during the reporting period.

 
Material Fund Change Strategies [Text Block] The fund modified its principal investment strategies during the reporting period.
 
Material Fund Change Risks Change [Text Block] The fund modified its principal investment risks during the reporting period.

 
Summary of Change Legend [Text Block] This is a summary of certain changes to the Fund since May 1, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by June 27, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-877-297-2952  or by sending an e-mail to fidfunddocuments@fidelity.com.  
Updated Prospectus Phone Number <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">1-877-297-2952</span>  
Updated Prospectus Email Address <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">fidfunddocuments@fidelity.com</span>  
Updated Prospectus Web Address <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:bold;">fundresearch.fidelity.com/prospectus/sec</span>  
Fidelity Treasury Only Money Market Fund - Class OUS    
Shareholder Report [Line Items]    
Fund Name Fidelity® Treasury Only Money Market Fund  
Class Name Fidelity® Treasury Only Money Market Fund Class OUS  
Trading Symbol FYIXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Treasury Only Money Market Fund for the period May 1, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 800-544-0275 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Material Fund Change Notice [Text Block] This report describes changes to the Fund that occurred during the reporting period.  
Additional Information Phone Number 800-544-0275  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Class OUS 
$ 38 
0.37%
  
 
Expenses Paid, Amount $ 38  
Expense Ratio, Percent 0.37%  
Net Assets $ 16,940,567,988 $ 16,940,567,988
Holdings Count | shares 57 57
Advisory Fees Paid, Amount $ 61,283,591  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$16,940,567,988
  
Number of Holdings
57
  
Total Advisory Fee
$61,283,591
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 19.9   8-30 17.9   31-60 24.1   61-90 21.0   91-180 16.4   >180 1.6             U.S. Treasury Obligations 100.9 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 100.9        Net Other Assets (Liabilities) - (0.9)%    
Material Fund Change [Text Block]
How has the Fund changed?
 
This is a summary of certain changes to the Fund since May 1, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by June 27, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 800-544-0275  or by sending an e-mail to fidfunddocuments@fidelity.com.
 
The fund modified its principal investment strategies during the reporting period.
The fund modified its principal investment risks during the reporting period.

 
Material Fund Change Strategies [Text Block] The fund modified its principal investment strategies during the reporting period.
 
Material Fund Change Risks Change [Text Block] The fund modified its principal investment risks during the reporting period.

 
Summary of Change Legend [Text Block] This is a summary of certain changes to the Fund since May 1, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by June 27, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 800-544-0275  or by sending an e-mail to fidfunddocuments@fidelity.com.  
Updated Prospectus Phone Number <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">800-544-0275</span>  
Updated Prospectus Email Address <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">fidfunddocuments@fidelity.com</span>  
Updated Prospectus Web Address <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:bold;">fundresearch.fidelity.com/prospectus/sec</span>  
Fidelity Treasury Digital Fund - OnChain Class    
Shareholder Report [Line Items]    
Fund Name Fidelity® Treasury Digital Fund  
Class Name Fidelity® Treasury Digital Fund OnChain  
Trading Symbol FYOXX  
Annual or Semi-Annual Statement [Text Block] This annual shareholder report contains information about Fidelity® Treasury Digital Fund for the period August 4, 2025 to April 30, 2026.  
Shareholder Report Annual or Semi-Annual annual shareholder report  
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-877-297-2952 or by sending an e-mail to fidfunddocuments@fidelity.com.  
Material Fund Change Notice [Text Block] This report describes changes to the Fund that occurred during the reporting period.  
Additional Information Phone Number 1-877-297-2952  
Additional Information Email fidfunddocuments@fidelity.com  
Additional Information Website fundresearch.fidelity.com/prospectus/sec  
Expenses [Text Block] What were your Fund costs for the last year?
(based on hypothetical $10,000 investment)
 
FUND COST (PREVIOUS YEAR)
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
OnChain A
$ 15 
0.20%
  
AExpenses for the full reporting period would be higher.
 
  
Expenses Paid, Amount $ 15  
Expense Ratio, Percent 0.20%  
Expenses Short Period Footnote [Text Block] Expenses for the full reporting period would be higher.  
Net Assets $ 171,679,853 $ 171,679,853
Holdings Count | shares 17 17
Advisory Fees Paid, Amount $ 621,595  
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of April 30, 2026)
 
KEY FACTS
 
  
  
Fund Size
$171,679,853
  
Number of Holdings
17
  
Total Advisory Fee
$621,595
  
 
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 10.5   8-30 24.2   31-60 38.5   61-90 31.5             U.S. Treasury Obligations 104.7 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 104.7        Net Other Assets (Liabilities) - (4.7)%    
Material Fund Change [Text Block]
How has the Fund changed?
 
This is a summary of certain changes to the Fund since August 4, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by June 27, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-877-297-2952  or by sending an e-mail to fidfunddocuments@fidelity.com.
 
The fund modified its principal investment strategies during the reporting period.
The fund modified its principal investment risks during the reporting period.

 
Material Fund Change Strategies [Text Block] The fund modified its principal investment strategies during the reporting period.
 
Material Fund Change Risks Change [Text Block] The fund modified its principal investment risks during the reporting period.

 
Summary of Change Legend [Text Block] This is a summary of certain changes to the Fund since August 4, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by June 27, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-877-297-2952  or by sending an e-mail to fidfunddocuments@fidelity.com.  
Updated Prospectus Phone Number <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">1-877-297-2952</span>  
Updated Prospectus Email Address <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">fidfunddocuments@fidelity.com</span>  
Updated Prospectus Web Address <span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:bold;">fundresearch.fidelity.com/prospectus/sec</span>