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Shareholder Report
 6 Months Ended
Jul. 31, 2025
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, 2025
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, 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 1-800-544-8544 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 $ 3,374,729,695
Holdings Count | shares 263
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$3,374,729,695
  
Number of Holdings
263
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 71.8   8-30 1.6   31-60 8.6   61-90 5.0   91-180 7.3   >180 4.5             Variable Rate Demand Notes (VRDNs) 32.6 Municipal Securities 25.8 Tender Option Bond 25.1 Commercial Paper 1.5 Net Other Assets (Liabilities) 15.0 ASSET ALLOCATION (% of Fund's net assets)     Variable Rate Demand Notes (VRDNs) - 32.6   Municipal Securities - 25.8                Tender Option Bond - 25.1                  Commercial Paper - 1.5                     Net Other Assets (Liabilities) - 15.0     
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, 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 1-800-544-8544 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 $ 3,374,729,695
Holdings Count | shares 263
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$3,374,729,695
  
Number of Holdings
263
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 71.8   8-30 1.6   31-60 8.6   61-90 5.0   91-180 7.3   >180 4.5             Variable Rate Demand Notes (VRDNs) 32.6 Municipal Securities 25.8 Tender Option Bond 25.1 Commercial Paper 1.5 Net Other Assets (Liabilities) 15.0 ASSET ALLOCATION (% of Fund's net assets)     Variable Rate Demand Notes (VRDNs) - 32.6   Municipal Securities - 25.8                Tender Option Bond - 25.1                  Commercial Paper - 1.5                     Net Other Assets (Liabilities) - 15.0     
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, 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 1-877-208-0098 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 $ 3,374,729,695
Holdings Count | shares 263
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$3,374,729,695
  
Number of Holdings
263
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 71.8   8-30 1.6   31-60 8.6   61-90 5.0   91-180 7.3   >180 4.5             Variable Rate Demand Notes (VRDNs) 32.6 Municipal Securities 25.8 Tender Option Bond 25.1 Commercial Paper 1.5 Net Other Assets (Liabilities) 15.0 ASSET ALLOCATION (% of Fund's net assets)     Variable Rate Demand Notes (VRDNs) - 32.6   Municipal Securities - 25.8                Tender Option Bond - 25.1                  Commercial Paper - 1.5                     Net Other Assets (Liabilities) - 15.0     
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, 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 1-800-544-8544 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 $ 1,688,472,916
Holdings Count | shares 463
Investment Company Portfolio Turnover 27.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of July 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$1,688,472,916
  
Number of Holdings
463
  
Portfolio Turnover
27%
  
Holdings [Text Block]   REVENUE SOURCES (% of Fund's net assets) General Obligations 33.5   Education 15.1   Special Tax 14.9   Transportation 11.6   Health Care 9.8   Water & Sewer 5.1   Others(Individually Less Than 5%) 5.5   95.5       AAA 10.0 AA 65.5 A 5.3 BBB 12.4 BB 0.4 Not Rated 1.9 Short-Term Investments and Net Other Assets (Liabilities) 4.5 QUALITY DIVERSIFICATION (% of Fund's net assets)     AAA - 10.0                                 AA - 65.5                                  A - 5.3                                    BBB - 12.4                                 BB - 0.4                                   Not Rated - 1.9                            Short-Term Investments and Net Other Assets (Liabilities) - 4.5   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.