v3.26.3
Shareholder Report
 6 Months Ended
Jul. 31, 2026
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, 2026
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, 2026 to July 31, 2026.
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 
0.30%
  
Expenses Paid, Amount $ 15
Expense Ratio, Percent 0.30%
Net Assets $ 243,673,728
Holdings Count | shares 596
Investment Company Portfolio Turnover 16.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$243,673,728
  
Number of Holdings
596
  
Portfolio Turnover
16%
  
Holdings [Text Block]   REVENUE SOURCES (% of Fund's net assets) General Obligations 27.6   Health Care 19.7   Transportation 13.6   Housing 10.8   Education 9.0   Special Tax 5.9   Others(Individually Less Than 5%) 10.3   96.9       AAA 5.5 AA 37.7 A 28.9 BBB 11.7 BB 2.1 Not Rated 11.0 Short-Term Investments and Net Other Assets (Liabilities) 3.1 QUALITY DIVERSIFICATION (% of Fund's net assets)     AAA - 5.5                                  AA - 37.7                                  A - 28.9                                   BBB - 11.7                                 BB - 2.1                                   Not Rated - 11.0                           Short-Term Investments and Net Other Assets (Liabilities) - 3.1   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.       TOP STATES (% of Fund's net assets) Texas 11.4 Alabama 10.6 Illinois 7.3 Florida 5.0 Pennsylvania 4.0