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Shareholder Report
 6 Months Ended
Jul. 31, 2024
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, 2024
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, 2024 to July 31, 2024.
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 $ 3,025,435,985
Holdings Count | shares 268
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of July 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$3,025,435,985
 
  
Number of Holdings
268
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 71.6   8-30 2.4   31-60 5.8   61-90 5.7   91-180 6.3   >180 7.2         Variable Rate Demand Notes (VRDNs) 33.9 Municipal Securities 28.2 Tender Option Bond 23.7 Commercial Paper 0.3 Net Other Assets (Liabilities) 13.9 ASSET ALLOCATION (% of Fund's net assets)
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, 2024 to July 31, 2024.
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 
$ 23 
0.45%
  
Expenses Paid, Amount $ 23
Expense Ratio, Percent 0.45%
Net Assets $ 1,722,695,148
Holdings Count | shares 465
Investment Company Portfolio Turnover 6.00%
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of July 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$1,722,695,148
 
  
Number of Holdings
465
 
  
Portfolio Turnover
6%
 
  
Holdings [Text Block]   REVENUE SOURCES (% of Fund's net assets) General Obligations 22.5   Education 21.9   Health Care 17.7   Transportation 13.0   Special Tax 12.7   Others(Individually Less Than 5%) 12.2   100.0       AAA 3.9 AA 62.0 A 10.8 BBB 16.7 BB 0.4 B 1.1 Not Rated 2.8 Short-Term Investments and Net Other Assets (Liabilities) 2.3 QUALITY DIVERSIFICATION (% of Fund's net assets)     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.    
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, 2024 to July 31, 2024.
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 $ 3,025,435,985
Holdings Count | shares 268
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of July 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$3,025,435,985
 
  
Number of Holdings
268
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 71.6   8-30 2.4   31-60 5.8   61-90 5.7   91-180 6.3   >180 7.2         Variable Rate Demand Notes (VRDNs) 33.9 Municipal Securities 28.2 Tender Option Bond 23.7 Commercial Paper 0.3 Net Other Assets (Liabilities) 13.9 ASSET ALLOCATION (% of Fund's net assets)
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, 2024 to July 31, 2024.
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 $ 3,025,435,985
Holdings Count | shares 268
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of July 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$3,025,435,985
 
  
Number of Holdings
268
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 71.6   8-30 2.4   31-60 5.8   61-90 5.7   91-180 6.3   >180 7.2         Variable Rate Demand Notes (VRDNs) 33.9 Municipal Securities 28.2 Tender Option Bond 23.7 Commercial Paper 0.3 Net Other Assets (Liabilities) 13.9 ASSET ALLOCATION (% of Fund's net assets)