v3.25.2
Shareholder Report
 6 Months Ended
Jul. 31, 2025
USD ($)
shares
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name Fidelity Merrimack Street Trust
Entity Central Index Key 0001562565
Entity Investment Company Type N-1A
Document Period End Date Jul. 31, 2025
Fidelity Municipal Bond Opportunities ETF  
Shareholder Report [Line Items]  
Fund Name Fidelity® Municipal Bond Opportunities ETF
Class Name Fidelity® Municipal Bond Opportunities ETF
Trading Symbol FMUB
Security Exchange Name NASDAQ
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Municipal Bond Opportunities ETF for the period February 1, 2025 to July 31, 2025.
Shareholder Report Annual or Semi-Annual semi-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.
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 six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Municipal Bond Opportunities ETF 
$ 15 
0.31%
  
Expenses Paid, Amount $ 15
Expense Ratio, Percent 0.31%
Net Assets $ 106,153,445
Holdings Count | shares 411
Investment Company Portfolio Turnover 50.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$106,153,445
  
Number of Holdings
411
  
Portfolio Turnover
50%
  
Holdings [Text Block]   REVENUE SOURCES (% of Fund's net assets) General Obligations 26.8   Transportation 16.1   Health Care 14.7   Education 9.0   Water & Sewer 8.1   Special Tax 6.9   Electric Utilities 5.0   Others(Individually Less Than 5%) 7.7   94.3       AAA 6.8 AA 37.5 A 30.7 BBB 13.1 BB 1.7 B 0.2 Not Rated 4.3 Short-Term Investments and Net Other Assets (Liabilities) 5.7 QUALITY DIVERSIFICATION (% of Fund's net assets)     AAA - 6.8                                  AA - 37.5                                  A - 30.7                                   BBB - 13.1                                 BB - 1.7                                   B - 0.2                                    Not Rated - 4.3                            Short-Term Investments and Net Other Assets (Liabilities) - 5.7   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) Illinois 11.1 Texas 10.6 Pennsylvania 8.0 Georgia 5.0 California 4.2