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 Massachusetts Municipal Trust
Entity Central Index Key 0000357057
Entity Investment Company Type N-1A
Document Period End Date Jul. 31, 2026
Fidelity Massachusetts Municipal Money Market Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Massachusetts Municipal Money Market Fund
Class Name Fidelity® Massachusetts Municipal Money Market Fund
Trading Symbol FAUXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Massachusetts Municipal Money Market Fund 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 1-800-544-8544 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 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® Massachusetts Municipal Money Market Fund 
$ 21 
0.42%
  
Expenses Paid, Amount $ 21
Expense Ratio, Percent 0.42%
Net Assets $ 3,982,131,065
Holdings Count | shares 259
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$3,982,131,065
  
Number of Holdings
259
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 74.7   8-30 0.7   31-60 5.2   61-90 5.2   91-180 4.0   >180 6.5             Variable Rate Demand Notes (VRDNs) 39.0 Municipal Securities 22.6 Tender Option Bond 21.5 Net Other Assets (Liabilities) 16.9 ASSET ALLOCATION (% of Fund's net assets)     Variable Rate Demand Notes (VRDNs) - 39.0   Municipal Securities - 22.6                Tender Option Bond - 21.5                  Net Other Assets (Liabilities) - 16.9     
Fidelity Massachusetts Municipal Money Market Fund - Premium Class  
Shareholder Report [Line Items]  
Fund Name Fidelity® Massachusetts Municipal Money Market Fund
Class Name Fidelity® Massachusetts Municipal Money Market Fund Premium Class
Trading Symbol FMSXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Massachusetts Municipal Money Market Fund 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 1-800-544-8544 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 six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Premium Class 
$ 15 
0.30%
  
Expenses Paid, Amount $ 15
Expense Ratio, Percent 0.30%
Net Assets $ 3,982,131,065
Holdings Count | shares 259
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$3,982,131,065
  
Number of Holdings
259
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 74.7   8-30 0.7   31-60 5.2   61-90 5.2   91-180 4.0   >180 6.5             Variable Rate Demand Notes (VRDNs) 39.0 Municipal Securities 22.6 Tender Option Bond 21.5 Net Other Assets (Liabilities) 16.9 ASSET ALLOCATION (% of Fund's net assets)     Variable Rate Demand Notes (VRDNs) - 39.0   Municipal Securities - 22.6                Tender Option Bond - 21.5                  Net Other Assets (Liabilities) - 16.9     
Fidelity Massachusetts Municipal Money Market Fund - Institutional Class  
Shareholder Report [Line Items]  
Fund Name Fidelity® Massachusetts Municipal Money Market Fund
Class Name Fidelity® Massachusetts Municipal Money Market Fund Institutional Class
Trading Symbol FMAXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Massachusetts Municipal Money Market Fund 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 1-877-208-0098 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-877-208-0098
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
 
Institutional Class 
$ 10 
0.20%
  
Expenses Paid, Amount $ 10
Expense Ratio, Percent 0.20%
Net Assets $ 3,982,131,065
Holdings Count | shares 259
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$3,982,131,065
  
Number of Holdings
259
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 74.7   8-30 0.7   31-60 5.2   61-90 5.2   91-180 4.0   >180 6.5             Variable Rate Demand Notes (VRDNs) 39.0 Municipal Securities 22.6 Tender Option Bond 21.5 Net Other Assets (Liabilities) 16.9 ASSET ALLOCATION (% of Fund's net assets)     Variable Rate Demand Notes (VRDNs) - 39.0   Municipal Securities - 22.6                Tender Option Bond - 21.5                  Net Other Assets (Liabilities) - 16.9     
Fidelity Massachusetts Municipal Income Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Massachusetts Municipal Income Fund
Class Name Fidelity® Massachusetts Municipal Income Fund
Trading Symbol FDMMX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Massachusetts Municipal Income Fund 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 1-800-544-8544 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 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® Massachusetts Municipal Income Fund 
$ 22 
0.45%
  
Expenses Paid, Amount $ 22
Expense Ratio, Percent 0.45%
Net Assets $ 1,622,881,034
Holdings Count | shares 467
Investment Company Portfolio Turnover 24.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$1,622,881,034
  
Number of Holdings
467
  
Portfolio Turnover
24%
  
Holdings [Text Block]   REVENUE SOURCES (% of Fund's net assets) General Obligations 32.0   Education 17.7   Transportation 12.7   Special Tax 11.0   Health Care 10.4   Water & Sewer 5.8   Others(Individually Less Than 5%) 7.2   96.8       AAA 9.5 AA 62.6 A 8.3 BBB 12.6 BB 1.8 Not Rated 2.0 Short-Term Investments and Net Other Assets (Liabilities) 3.2 QUALITY DIVERSIFICATION (% of Fund's net assets)     AAA - 9.5                                  AA - 62.6                                  A - 8.3                                    BBB - 12.6                                 BB - 1.8                                   Not Rated - 2.0                            Short-Term Investments and Net Other Assets (Liabilities) - 3.2   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.