v3.26.3
Risk/Return Detail Data - FidelityArizonaMunicipalIncomeFund-PRO - USD ($)
12 Months Ended 60 Months Ended 120 Months Ended
Oct. 30, 2026
Oct. 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
Oct. 30, 2026
Oct. 30, 2026
FidelityArizonaMunicipalIncomeFund-PRO | Fidelity Arizona Municipal Income Fund | Fidelity Arizona Municipal Income Fund                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.55%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.00%                          
Expenses (as a percentage of Assets) 0.55%                          
Expense Example, with Redemption, 1 Year $ 56                          
Expense Example, with Redemption, 3 Years 176                          
Expense Example, with Redemption, 5 Years 307                          
Expense Example, with Redemption, 10 Years $ 689                          
Average Annual Return, Label [Optional Text]   Return Before Taxes                        
Average Annual Return, Percent   4.05%                     0.74% 2.12%
Annual Return [Percent]     4.05% 1.83% 6.02% (8.73%) 1.21% 4.50% 7.23% 0.63% 5.43% none    
FidelityArizonaMunicipalIncomeFund-PRO | Fidelity Arizona Municipal Income Fund                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund: Fidelity® Arizona Municipal Income Fund                          
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Fidelity® Arizona Municipal Income Fund seeks a high level of current income, exempt from federal income tax and Arizona personal income tax.                          
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, 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                          
Portfolio Turnover [Heading] Portfolio Turnover                          
Portfolio Turnover [Text Block] The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when fund shares are held in a taxable account. These costs, which are not reflected in annual operating expenses or in the example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 9 % of the average value of its portfolio.                          
Portfolio Turnover, Rate 9.00%                          
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 80% of assets in municipal securities whose interest is exempt from federal and Arizona personal income taxes. The municipal securities in which the fund invests are normally investment-grade (those of medium and high quality). Municipal debt 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. Municipal debt securities in which the fund invests include securities issued by U.S. territories and possessions, general obligation bonds of municipalities, local or state governments, project or revenue-specific bonds, or pre-refunded or escrowed bonds, municipal money market securities, and synthetic securities. Municipal securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. Investment grade securities in which the fund invests include securities or issuers rated at least BBB- or its equivalent by at least one nationally recognized statistical ratings organization (NRSRO), or, if unrated, determined by the Adviser to be of comparable quality. Derivative instruments that provide investment exposure to the investments above or exposure to one or more market risk factors associated with such investments are included in the fund's 80% policy, consistent with the fund's investment policies and limitations with respect to investments in derivatives. Managing the fund to have similar overall interest rate risk to the supplemental index (see "Additional Index Information" section of the prospectus). Allocating assets across different market sectors and maturities. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select 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 and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time. The additional index has characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance. Visit www.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 and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time.                          
Performance Availability Website Address [Text] www.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance (before and after taxes) 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 Sep. 30, 2026                          
Bar Chart, Year to Date Return (3.79%)                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 7.25%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return (5.93%)                          
Performance Table Heading Average Annual Returns                          
Performance Table Uses Highest Federal Rate After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes.                          
Performance Table Explanation after Tax Higher Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.                          
Performance Table Narrative After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.                          
FidelityArizonaMunicipalIncomeFund-PRO | Fidelity Arizona Municipal Income Fund | After Taxes on Distributions | Fidelity Arizona Municipal Income Fund                            
Prospectus Line Items                            
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions                        
Average Annual Return, Percent   4.05%                     0.73% 2.08%
FidelityArizonaMunicipalIncomeFund-PRO | Fidelity Arizona Municipal Income Fund | After Taxes on Distributions and Sales | Fidelity Arizona Municipal Income Fund                            
Prospectus Line Items                            
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions and Sale of Fund Shares                        
Average Annual Return, Percent   3.50%                     1.10% 2.19%
FidelityArizonaMunicipalIncomeFund-PRO | Fidelity Arizona Municipal Income Fund | LB015                            
Prospectus Line Items                            
Average Annual Return, Label [Optional Text]   Bloomberg Municipal Bond Index                        
Average Annual Return, Percent   4.25%                     0.80% 2.34%
FidelityArizonaMunicipalIncomeFund-PRO | Fidelity Arizona Municipal Income Fund | IXYGL                            
Prospectus Line Items                            
Average Annual Return, Label [Optional Text]   Bloomberg Arizona 2+ Year Enhanced Municipal Linked Index                        
Average Annual Return, Percent   4.54%                     0.71% 2.30%
FidelityMarylandMunicipalIncomeFund-PRO | Fidelity Maryland Municipal Income Fund | Fidelity Maryland Municipal Income Fund                            
Prospectus Line Items                            
Management Fees (as a percentage of Assets) 0.55%                          
Distribution and Service (12b-1) Fees 0.00%                          
Other Expenses (as a percentage of Assets): 0.00%                          
Expenses (as a percentage of Assets) 0.55%                          
Expense Example, with Redemption, 1 Year $ 56                          
Expense Example, with Redemption, 3 Years 176                          
Expense Example, with Redemption, 5 Years 307                          
Expense Example, with Redemption, 10 Years $ 689                          
Average Annual Return, Label [Optional Text]   Return Before Taxes                        
Average Annual Return, Percent   4.94%                     0.97% 2.14%
Annual Return [Percent]     4.94% 1.35% 6.20% (8.88%) 1.96% 3.78% 6.96% 0.26% 5.92% (0.13%)    
FidelityMarylandMunicipalIncomeFund-PRO | Fidelity Maryland Municipal Income Fund                            
Prospectus Line Items                            
Risk/Return [Heading] Fund Summary Fund: Fidelity® Maryland Municipal Income Fund                          
Objective [Heading] Investment Objective                          
Objective, Primary [Text Block] Fidelity® Maryland Municipal Income Fund seeks a high level of current income, exempt from federal income tax and Maryland personal income taxes.                          
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, 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                          
Portfolio Turnover [Heading] Portfolio Turnover                          
Portfolio Turnover [Text Block] The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when fund shares are held in a taxable account. These costs, which are not reflected in annual operating expenses or in the example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 5 % of the average value of its portfolio.                          
Portfolio Turnover, Rate 5.00%                          
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 80% of assets in municipal securities whose interest is exempt from federal and Maryland personal income taxes. The municipal securities in which the fund invests are normally investment-grade (those of medium and high quality). Municipal debt 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. Municipal debt securities in which the fund invests include securities issued by U.S. territories and possessions, general obligation bonds of municipalities, local or state governments, project or revenue-specific bonds, or pre-refunded or escrowed bonds, municipal money market securities, and synthetic securities. Municipal securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. Investment grade securities in which the fund invests include securities or issuers rated at least BBB- or its equivalent by at least one nationally recognized statistical ratings organization (NRSRO), or, if unrated, determined by the Adviser to be of comparable quality. Derivative instruments that provide investment exposure to the investments above or exposure to one or more market risk factors associated with such investments are included in the fund's 80% policy, consistent with the fund's investment policies and limitations with respect to investments in derivatives. Managing the fund to have similar overall interest rate risk to the supplemental index (see "Additional Index Information" section of the prospectus). Allocating assets across different market sectors and maturities. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select 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 and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time. The additional index has characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance. Visit www.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 and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time.                          
Performance Availability Website Address [Text] www.fidelity.com                          
Performance Past Does Not Indicate Future [Text] Past performance (before and after taxes) 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 Sep. 30, 2026                          
Bar Chart, Year to Date Return (4.73%)                          
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                          
Highest Quarterly Return, Date Dec. 31, 2023                          
Highest Quarterly Return 7.81%                          
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                          
Lowest Quarterly Return, Date Mar. 31, 2022                          
Lowest Quarterly Return (5.91%)                          
Performance Table Heading Average Annual Returns                          
Performance Table Uses Highest Federal Rate After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes.                          
Performance Table Explanation after Tax Higher Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.                          
Performance Table Narrative After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.                          
FidelityMarylandMunicipalIncomeFund-PRO | Fidelity Maryland Municipal Income Fund | After Taxes on Distributions | Fidelity Maryland Municipal Income Fund                            
Prospectus Line Items                            
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions                        
Average Annual Return, Percent   4.94%                     0.95% 2.10%
FidelityMarylandMunicipalIncomeFund-PRO | Fidelity Maryland Municipal Income Fund | After Taxes on Distributions and Sales | Fidelity Maryland Municipal Income Fund                            
Prospectus Line Items                            
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions and Sale of Fund Shares                        
Average Annual Return, Percent   4.01%                     1.27% 2.18%
FidelityMarylandMunicipalIncomeFund-PRO | Fidelity Maryland Municipal Income Fund | IXYH6                            
Prospectus Line Items                            
Average Annual Return, Label [Optional Text]   Bloomberg Maryland 2+ Year Enhanced Municipal Linked Index                        
Average Annual Return, Percent   4.69%                     0.51% 2.18%
FidelityMarylandMunicipalIncomeFund-PRO | Fidelity Maryland Municipal Income Fund | LB015                            
Prospectus Line Items                            
Average Annual Return, Label [Optional Text]   Bloomberg Municipal Bond Index                        
Average Annual Return, Percent   4.25%                     0.80% 2.34%
Document Type 485BPOS                          
Registrant Name Fidelity Union Street Trust