Shareholder Report |
12 Months Ended | |||||||||||||||
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Aug. 31, 2026
USD ($)
shares
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| Shareholder Report [Line Items] | ||||||||||||||||
| Document Type | N-CSR | |||||||||||||||
| Amendment Flag | false | |||||||||||||||
| Registrant Name | Fidelity Union Street Trust | |||||||||||||||
| Entity Central Index Key | 0000035330 | |||||||||||||||
| Entity Investment Company Type | N-1A | |||||||||||||||
| Document Period End Date | Aug. 31, 2026 | |||||||||||||||
| Fidelity Maryland Municipal Income Fund | ||||||||||||||||
| Shareholder Report [Line Items] | ||||||||||||||||
| Fund Name | Fidelity® Maryland Municipal Income Fund | |||||||||||||||
| Class Name | Fidelity® Maryland Municipal Income Fund | |||||||||||||||
| Trading Symbol | SMDMX | |||||||||||||||
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains information about Fidelity® Maryland Municipal Income Fund for the period September 1, 2025 to August 31, 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)
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| Expenses Paid, Amount | $ 56 | |||||||||||||||
| Expense Ratio, Percent | 0.55% | |||||||||||||||
| Factors Affecting Performance [Text Block] | What affected the Fund's performance this period?
•Tax-exempt municipal bonds achieved a solid gain for the 12 months ending August 31, 2026, buoyed by three policy interest-rate cuts by the Federal Reserve in late 2025, but pulled down since then by rising bond yields amid higher consumer inflation, fast-growing government debt and heavy U.S. Treasury issuance.
•Against this backdrop, the fund's "carry" advantage, meaning its proportionately larger stake in higher-yielding bonds, contributed to performance versus the Bloomberg Maryland 2+ Year Enhanced Municipal Linked 08/01/2018 Index for the fiscal year.
•Yield-curve positioning, or how the fund's investments were allocated across bonds with various maturities, contributed, as did an overweight in lower-quality investment-grade municipal bonds.
•In contrast, overweights in certain high-quality bonds detracted versus the state index this period.
•There were no major changes to the fund from the perspective of sector allocation, yield-curve positioning and credit-quality exposure during the past 12 months.
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| 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 past 10 years? CUMULATIVE PERFORMANCE August 31, 2016 through August 31, 2026. Initial investment of $10,000. Fidelity® Maryland Municipal Income Fund $10,000 $10,033 $10,017 $10,866 $11,024 $11,474 $10,424 $10,584 $11,243 $11,315 $11,731 Bloomberg Maryland 2+ Year Enhanced Municipal Linked Index $10,000 $10,092 $10,076 $10,934 $11,350 $11,611 $10,631 $10,730 $11,323 $11,386 $11,795 Bloomberg Municipal Bond Index $10,000 $10,088 $10,138 $11,022 $11,378 $11,765 $10,749 $10,932 $11,598 $11,607 $12,085 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 | |||||||||||||||
| Average Annual Return [Table Text Block] | AVERAGE ANNUAL TOTAL RETURNS: 1 Year 5 Year 10 Year Fidelity® Maryland Municipal Income Fund 3.68% 0.45% 1.61% Bloomberg Maryland 2+ Year Enhanced Municipal Linked Index 3.58% 0.31% 1.66% Bloomberg Municipal Bond Index 4.12% 0.54% 1.91% 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. | |||||||||||||||
| 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 | $ 161,193,747 | |||||||||||||||
| Holdings Count | shares | 155 | |||||||||||||||
| Advisory Fees Paid, Amount | $ 876,445 | |||||||||||||||
| Investment Company Portfolio Turnover | 5.00% | |||||||||||||||
| Additional Fund Statistics [Text Block] | Key Fund Statistics
(as of August 31, 2026)
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| Holdings [Text Block] | REVENUE SOURCES (% of Fund's net assets) General Obligations 39.2 Health Care 19.2 Special Tax 12.0 Transportation 9.7 Water & Sewer 7.8 Housing 6.1 Others(Individually Less Than 5%) 4.7 98.7 AAA 27.7 AA 32.2 A 19.6 BBB 11.9 B 0.9 Not Rated 6.4 Short-Term Investments and Net Other Assets (Liabilities) 1.3 QUALITY DIVERSIFICATION (% of Fund's net assets) AAA - 27.7 AA - 32.2 A - 19.6 BBB - 11.9 B - 0.9 Not Rated - 6.4 Short-Term Investments and Net Other Assets (Liabilities) - 1.3 We have used ratings from Moody's Investors Service, Inc. Where Moody's® ratings are not available, we have used S&P® ratings. All ratings are as of the date indicated and do not reflect subsequent changes. | |||||||||||||||
| Fidelity Arizona Municipal Income Fund | ||||||||||||||||
| Shareholder Report [Line Items] | ||||||||||||||||
| Fund Name | Fidelity® Arizona Municipal Income Fund | |||||||||||||||
| Class Name | Fidelity® Arizona Municipal Income Fund | |||||||||||||||
| Trading Symbol | FSAZX | |||||||||||||||
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains information about Fidelity® Arizona Municipal Income Fund for the period September 1, 2025 to August 31, 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)
|
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| Expenses Paid, Amount | $ 56 | |||||||||||||||
| Expense Ratio, Percent | 0.55% | |||||||||||||||
| Factors Affecting Performance [Text Block] | What affected the Fund's performance this period?
•Tax-exempt municipal bonds achieved a solid gain for the 12 months ending August 31, 2026, buoyed by three policy interest-rate cuts by the Federal Reserve in late 2025, but pulled down since then by rising bond yields amid higher consumer inflation, fast-growing government debt and heavy U.S. Treasury issuance.
•Against this backdrop, the fund's underweight in certain high-quality municipal bonds contributed to performance versus the Bloomberg Arizona 2+ Year Enhanced Municipal Linked 08/01/2018 Index for the fiscal year.
•The fund's "carry" advantage, meaning its proportionately larger stake in higher-yielding bonds, helped.
•Pricing factors meaningfully contributed. Fund holdings are priced by a third-party pricing service and validated daily by Fidelity Management & Research's fair-value processes. Securities within the index, however, are priced by the index provider. These two approaches employ somewhat different methodologies in setting the prices of municipal securities, most of which trade infrequently.
•In contrast, an overweight in certain high-quality bonds that lagged the state index detracted from the fund's relative result this period.
•There were no major changes to the fund from the perspective of sector allocation, yield-curve positioning and credit-quality exposure during the past 12 months.
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| 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 past 10 years? CUMULATIVE PERFORMANCE August 31, 2016 through August 31, 2026. Initial investment of $10,000. Fidelity® Arizona Municipal Income Fund $10,000 $10,043 $10,037 $10,897 $11,151 $11,516 $10,472 $10,630 $11,351 $11,343 $11,782 Bloomberg Arizona 2+ Year Enhanced Municipal Linked Index $10,000 $10,123 $10,135 $11,018 $11,376 $11,721 $10,728 $10,847 $11,496 $11,569 $11,986 Bloomberg Municipal Bond Index $10,000 $10,088 $10,138 $11,022 $11,378 $11,765 $10,749 $10,932 $11,598 $11,607 $12,085 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 | |||||||||||||||
| Average Annual Return [Table Text Block] | AVERAGE ANNUAL TOTAL RETURNS: 1 Year 5 Year 10 Year Fidelity® Arizona Municipal Income Fund 3.86% 0.46% 1.65% Bloomberg Arizona 2+ Year Enhanced Municipal Linked Index 3.61% 0.45% 1.83% Bloomberg Municipal Bond Index 4.12% 0.54% 1.91% 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. | |||||||||||||||
| 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 | $ 134,576,162 | |||||||||||||||
| Holdings Count | shares | 146 | |||||||||||||||
| Advisory Fees Paid, Amount | $ 688,685 | |||||||||||||||
| Investment Company Portfolio Turnover | 9.00% | |||||||||||||||
| Additional Fund Statistics [Text Block] | Key Fund Statistics
(as of August 31, 2026)
|
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| Holdings [Text Block] | REVENUE SOURCES (% of Fund's net assets) General Obligations 31.0 Education 19.0 Health Care 16.9 Special Tax 6.6 Transportation 6.5 Electric Utilities 6.2 Water & Sewer 5.5 Others(Individually Less Than 5%) 3.2 94.9 AAA 5.2 AA 63.2 A 20.9 BBB 2.9 BB 0.4 Not Rated 2.3 Short-Term Investments and Net Other Assets (Liabilities) 5.1 QUALITY DIVERSIFICATION (% of Fund's net assets) AAA - 5.2 AA - 63.2 A - 20.9 BBB - 2.9 BB - 0.4 Not Rated - 2.3 Short-Term Investments and Net Other Assets (Liabilities) - 5.1 We have used ratings from Moody's Investors Service, Inc. Where Moody's® ratings are not available, we have used S&P® ratings. All ratings are as of the date indicated and do not reflect subsequent changes. |