Fidelity Massachuseets Municipal Income Fund (FDMMX)
Company Profile
| Form N-CSR Cover | 6 Months Ended |
| Jul. 31, 2024 | |
| 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 |
| 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 $ 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 $ 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 $ 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% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics (as of July 31, 2024) KEY FACTS Fund Size $ 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 $ 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 $ 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 $ 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 $ 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) |
| Shareholder Report, Holdings (Details) | Jul. 31, 2024 |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | EMD0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 71.60% |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | EMD1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | EMD2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.80% |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | EMD3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | EMD4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | EMD5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.20% |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | AAVariableRateDemandNotesVRDNsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 33.90% |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | AAMunicipalSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.20% |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | AATenderOptionBondMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.70% |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | AACommercialPaperMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.30% |
| Fidelity Massachusetts Municipal Money Market Fund - Institutional Class | AANetOtherAssetsLiabilitiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.90% |
| Fidelity Massachusetts Municipal Income Fund | RS0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 22.50% |
| Fidelity Massachusetts Municipal Income Fund | RS1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 21.90% |
| Fidelity Massachusetts Municipal Income Fund | RS2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 17.70% |
| Fidelity Massachusetts Municipal Income Fund | RS3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13% |
| Fidelity Massachusetts Municipal Income Fund | RS4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.70% |
| Fidelity Massachusetts Municipal Income Fund | RS5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.20% |
| Fidelity Massachusetts Municipal Income Fund | QDAMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 10.80% |
| Fidelity Massachusetts Municipal Income Fund | QDBBBMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 16.70% |
| Fidelity Massachusetts Municipal Income Fund | QDBBMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| Fidelity Massachusetts Municipal Income Fund | QDBMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.10% |
| Fidelity Massachusetts Municipal Income Fund | QDNotRatedMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.80% |
| Fidelity Massachusetts Municipal Income Fund | QDShortTermInvestmentsandNetOtherAssetsLiabilitiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.30% |
| Fidelity Massachusetts Municipal Money Market Fund | EMD0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 71.60% |
| Fidelity Massachusetts Municipal Money Market Fund | EMD1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| Fidelity Massachusetts Municipal Money Market Fund | EMD2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.80% |
| Fidelity Massachusetts Municipal Money Market Fund | EMD3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| Fidelity Massachusetts Municipal Money Market Fund | EMD4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| Fidelity Massachusetts Municipal Money Market Fund | EMD5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.20% |
| Fidelity Massachusetts Municipal Money Market Fund | AAVariableRateDemandNotesVRDNsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 33.90% |
| Fidelity Massachusetts Municipal Money Market Fund | AAMunicipalSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.20% |
| Fidelity Massachusetts Municipal Money Market Fund | AATenderOptionBondMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.70% |
| Fidelity Massachusetts Municipal Money Market Fund | AACommercialPaperMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.30% |
| Fidelity Massachusetts Municipal Money Market Fund | AANetOtherAssetsLiabilitiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.90% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | EMD0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 71.60% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | EMD1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.40% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | EMD2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.80% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | EMD3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.70% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | EMD4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.30% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | EMD5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.20% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | AAVariableRateDemandNotesVRDNsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 33.90% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | AAMunicipalSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 28.20% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | AATenderOptionBondMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 23.70% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | AACommercialPaperMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.30% |
| Fidelity Massachusetts Municipal Money Market Fund - Premium Class | AANetOtherAssetsLiabilitiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.90% |
| Label | Element | Value |
| C000019443 [Member] | QDAAMember [Member] | ||
| Percent of Net Asset Value | oef_PctOfNav | 62% |
| C000019443 [Member] | QDAAAMember [Member] | ||
| Percent of Net Asset Value | oef_PctOfNav | 3.90% |