Shareholder Report |
12 Months Ended | 25 Months Ended | 84 Months Ended | |||||||||||||||
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Jun. 30, 2026
USD ($)
shares
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Jun. 30, 2026
USD ($)
shares
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Jun. 30, 2026
USD ($)
shares
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| Shareholder Report [Line Items] | ||||||||||||||||||
| Document Type | N-CSR | |||||||||||||||||
| Amendment Flag | false | |||||||||||||||||
| Registrant Name | Fidelity Merrimack Street Trust | |||||||||||||||||
| Entity Central Index Key | 0001562565 | |||||||||||||||||
| Entity Investment Company Type | N-1A | |||||||||||||||||
| Document Period End Date | Jun. 30, 2026 | |||||||||||||||||
| Fidelity Systematic Municipal Bond Index ETF | ||||||||||||||||||
| Shareholder Report [Line Items] | ||||||||||||||||||
| Fund Name | Fidelity® Systematic Municipal Bond Index ETF | |||||||||||||||||
| Class Name | Fidelity® Systematic Municipal Bond Index ETF | |||||||||||||||||
| Trading Symbol | FMUN | |||||||||||||||||
| Security Exchange Name | NASDAQ | |||||||||||||||||
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains information about Fidelity® Systematic Municipal Bond Index ETF for the period July 1, 2025 to June 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-FIDELITY 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-FIDELITY | |||||||||||||||||
| 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 | $ 5 | |||||||||||||||||
| Expense Ratio, Percent | 0.05% | |||||||||||||||||
| Factors Affecting Performance [Text Block] | What affected the Fund's performance this period?
•Tax-exempt municipal bonds achieved a strong gain for the 12 months ending June 30, 2026, navigating uncertainty related to U.S. trade policy and geopolitical events but buoyed by attractive starting yields, three policy interest-rate cuts by the Federal Reserve in late 2025, and robust investor demand for the asset class that more than offset record supply of new muni bonds.
•We construct the exchange-traded fund using optimization. This approach minimizes the differences between risk exposures of the fund relative to the supplemental index, the Fidelity Systematic U.S. Municipal Bond 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.
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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 life of Fund? CUMULATIVE PERFORMANCE July 11, 2019 through June 30, 2026. Initial investment of $10,000. Fidelity® Systematic Municipal Bond Index ETF $10,146 $10,518 $9,544 $9,796 $10,091 $10,163 $10,854 Fidelity Systematic U.S. Municipal Bond index $10,146 $10,518 $9,544 $9,796 $10,075 $10,199 $10,897 Bloomberg Municipal Bond Index $10,410 $10,843 $9,914 $10,230 $10,558 $10,676 $11,427 2020 2021 2022 2023 2024 2025 2026 | |||||||||||||||||
| Average Annual Return [Table Text Block] | AVERAGE ANNUAL TOTAL RETURNS: 1 Year 5 Year Life of Fund B Fidelity® Systematic Municipal Bond Index ETF - NAV A 6.79% 0.63% 1.18% Fidelity Systematic U.S. Municipal Bond indexC 6.85% n/a n/a Bloomberg Municipal Bond Index 7.03% 1.05% 1.93% A Returns prior to the conversion are those of the former corresponding fundB From July 11, 2019 C From May 30, 2024 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 | May 30, 2024 | Jul. 11, 2019 | ||||||||||||||||
| 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 | $ 183,094,009 | $ 183,094,009 | $ 183,094,009 | |||||||||||||||
| Holdings Count | shares | 1,204 | 1,204 | 1,204 | |||||||||||||||
| Advisory Fees Paid, Amount | $ 80,259 | |||||||||||||||||
| Investment Company Portfolio Turnover | 15.00% | |||||||||||||||||
| Additional Fund Statistics [Text Block] | Key Fund Statistics
(as of June 30, 2026)
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| Holdings [Text Block] | REVENUE SOURCES (% of Fund's net assets) General Obligations 35.9 Transportation 16.1 Special Tax 13.7 Health Care 11.3 Water & Sewer 6.5 Education 5.7 Electric Utilities 5.1 Others(Individually Less Than 5%) 4.2 98.5 AAA 9.1 AA 58.3 A 28.1 BBB 3.0 BB 0.0 Not Rated 0.0 Short-Term Investments and Net Other Assets (Liabilities) 1.5 QUALITY DIVERSIFICATION (% of Fund's net assets) AAA - 9.1 AA - 58.3 A - 28.1 BBB - 3.0 BB - 0.0 Not Rated - 0.0 Short-Term Investments and Net Other Assets (Liabilities) - 1.5 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. TOP STATES (% of Fund's net assets) California 21.3 New York 17.4 Texas 8.2 Illinois 4.7 Pennsylvania 4.3 | |||||||||||||||||
| Material Fund Change [Text Block] |
How has the Fund changed?
This is a summary of certain changes to the Fund since July 1, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by October 30, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-800-FIDELITY or by sending an e-mail to fidfunddocuments@fidelity.com.
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| Material Fund Change Expenses [Text Block] | The fees associated with this class changed during the reporting year. The variations in class fees are primarily the result of the following changes:
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| Summary of Change Legend [Text Block] | This is a summary of certain changes to the Fund since July 1, 2025. For more complete information, you may review the Fund's next prospectus, which we expect to be available by October 30, 2026 at fundresearch.fidelity.com/prospectus/sec or upon request at 1-800-FIDELITY 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-FIDELITY</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> |