v3.26.1
Risk/Return Detail Data - FidelityMassachusettsMunicipalMoneyMarketFund-InstitutionalPRO - USD ($)
 12 Months Ended 48 Months Ended 60 Months Ended 120 Months Ended
Apr. 01, 2026
Apr. 01, 2026
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Apr. 01, 2026
 [2]
Apr. 01, 2026
Apr. 01, 2026
FidelityMassachusettsMunicipalMoneyMarketFund-InstitutionalPRO | Fidelity Massachusetts Municipal Money Market Fund | Fidelity Massachusetts Municipal Money Market Fund - Institutional Class                              
Prospectus Line Items                              
Management Fees (as a percentage of Assets) 0.20%                            
Distribution and Service (12b-1) Fees 0.00%                            
Other Expenses (as a percentage of Assets): 0.05%                            
Expenses (as a percentage of Assets) 0.25%                            
Fee Waiver or Reimbursement [1] (0.05%)                            
Net Expenses (as a percentage of Assets) 0.20%                            
Expense Example, with Redemption, 1 Year $ 20                            
Expense Example, with Redemption, 3 Years 64                            
Expense Example, with Redemption, 5 Years 113                            
Expense Example, with Redemption, 10 Years $ 255                            
Average Annual Return, Label [Optional Text]   Institutional Class                          
Average Annual Return, Percent   2.50%                       1.90% 1.34%
Annual Return [Percent]     2.50% 3.11% 2.96% 0.96% 0.02% 0.43% 1.31% 1.23% 0.66% 0.31%      
FidelityMassachusettsMunicipalMoneyMarketFund-InstitutionalPRO | Fidelity Massachusetts Municipal Money Market Fund                              
Prospectus Line Items                              
Risk/Return [Heading] Fund Summary Fund /Class: Fidelity® Massachusetts Municipal Money Market Fund /Institutional                              
Objective [Heading] Investment Objective                            
Objective, Primary [Text Block] Fidelity® Massachusetts Municipal Money Market Fund seeks as high a level of income, exempt from federal and Massachusetts personal income tax, as is consistent with the preservation of capital and liquidity.                            
Expense Heading [Optional Text] Fee Table                            
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                            
Shareholder Fees Caption [Optional Text] Shareholder fees                            
Shareholder Fee, Other $ 0                            
Operating Expenses Caption [Optional Text] Annual Operating Expenses                            
Fee Waiver or Reimbursement over Assets, Date of Termination will remain in effect for at least one year from the effective date of the prospectus                            
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                            
Strategy [Heading] Principal Investment Strategies                            
Strategy Narrative [Text Block] Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal and Massachusetts personal income taxes. Municipal money market securities are high-quality, short-term debt securities that pay a fixed, variable, or floating interest rate, and include variable rate demand notes, commercial paper, and municipal notes. Municipal securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Normally investing in municipal money market securities. Potentially investing up to 20% of assets in municipal securities whose interest is subject to Massachusetts personal income tax. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                            
Bar Chart and Performance Table [Heading] Performance                            
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.                            
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                            
Performance Availability Website Address [Text] www.fidelity.com or institutional.fidelity.com                            
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                            
Bar Chart [Heading] Year-by-Year Returns                            
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                            
Highest Quarterly Return, Date Jun. 30, 2024                            
Highest Quarterly Return 0.83%                            
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                            
Lowest Quarterly Return, Date Mar. 31, 2021                            
Lowest Quarterly Return none                            
Performance Table Heading Average Annual Returns                            
FidelityMassachusettsMunicipalIncomeandMunicipalMoneyMarketFund-RetailComboPRO | Fidelity Massachusetts Municipal Money Market Fund | Fidelity Massachusetts Municipal Money Market Fund                              
Prospectus Line Items                              
Management Fees (as a percentage of Assets) 0.20%                            
Distribution and Service (12b-1) Fees 0.00%                            
Other Expenses (as a percentage of Assets): 0.22%                            
Expenses (as a percentage of Assets) 0.42%                            
Expense Example, with Redemption, 1 Year $ 43                            
Expense Example, with Redemption, 3 Years 135                            
Expense Example, with Redemption, 5 Years 235                            
Expense Example, with Redemption, 10 Years $ 530                            
Annual Return, Inception Date Sep. 22, 2022                            
Average Annual Return, Label [Optional Text]   Fidelity® Massachusetts Municipal Money Market Fund                          
Average Annual Return, Percent   2.27%                     2.56%    
Annual Return [Percent]     2.27% 2.88% 2.73%                    
FidelityMassachusettsMunicipalIncomeandMunicipalMoneyMarketFund-RetailComboPRO | Fidelity Massachusetts Municipal Money Market Fund                              
Prospectus Line Items                              
Risk/Return [Heading] Fund Summary Fund /Class: Fidelity® Massachusetts Municipal Money Market Fund /Fidelity® Massachusetts Municipal Money Market Fund                            
Objective [Heading] Investment Objective                            
Objective, Primary [Text Block] Fidelity® Massachusetts Municipal Money Market Fund seeks as high a level of income, exempt from federal and Massachusetts personal income tax, as is consistent with the preservation of capital and liquidity.                            
Expense Heading [Optional Text] Fee Table                            
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                            
Shareholder Fees Caption [Optional Text] Shareholder fees                            
Shareholder Fee, Other $ 0                            
Operating Expenses Caption [Optional Text] Annual Operating Expenses                            
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                            
Strategy [Heading] Principal Investment Strategies                            
Strategy Narrative [Text Block] Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal and Massachusetts personal income taxes. Municipal money market securities are high-quality, short-term debt securities that pay a fixed, variable, or floating interest rate, and include variable rate demand notes, commercial paper, and municipal notes. Municipal securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Normally investing in municipal money market securities. Potentially investing up to 20% of assets in municipal securities whose interest is subject to Massachusetts personal income tax. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                            
Bar Chart and Performance Table [Heading] Performance                            
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com for more recent performance information.                            
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                            
Performance Availability Website Address [Text] www.fidelity.com                            
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                            
Bar Chart [Heading] Year-by-Year Returns                            
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                            
Highest Quarterly Return, Date Jun. 30, 2024                            
Highest Quarterly Return 0.77%                            
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                            
Lowest Quarterly Return, Date Mar. 31, 2025                            
Lowest Quarterly Return 0.54%                            
Performance Table Heading Average Annual Returns                            
FidelityMassachusettsMunicipalIncomeandMunicipalMoneyMarketFund-RetailComboPRO | Fidelity Massachusetts Municipal Income Fund | Fidelity Massachusetts Municipal Income Fund                              
Prospectus Line Items                              
Management Fees (as a percentage of Assets) 0.44%                            
Distribution and Service (12b-1) Fees 0.00%                            
Other Expenses (as a percentage of Assets): 0.00%                            
Expenses (as a percentage of Assets) 0.44%                            
Expense Example, with Redemption, 1 Year $ 45                            
Expense Example, with Redemption, 3 Years 141                            
Expense Example, with Redemption, 5 Years 246                            
Expense Example, with Redemption, 10 Years $ 555                            
Average Annual Return, Label [Optional Text]   Return Before Taxes                          
Average Annual Return, Percent   4.31%                       0.60% 1.98%
Annual Return [Percent]     4.31% 1.25% 5.75% (9.05%) 1.44% 4.44% 7.08% 0.24% 5.48% (0.11%)      
FidelityMassachusettsMunicipalIncomeandMunicipalMoneyMarketFund-RetailComboPRO | Fidelity Massachusetts Municipal Income Fund                              
Prospectus Line Items                              
Risk/Return [Heading] Fund Summary Fund: Fidelity® Massachusetts Municipal Income Fund                            
Objective [Heading] Investment Objective                            
Objective, Primary [Text Block] Fidelity® Massachusetts Municipal Income Fund seeks a high level of current income exempt from federal income tax and Massachusetts personal income tax.                            
Expense Heading [Optional Text] Fee Table                            
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions, which are not reflected in the tables and examples below.                            
Shareholder Fees Caption [Optional Text] Shareholder fees                            
Shareholder Fee, Other $ 0                            
Operating Expenses Caption [Optional Text] Annual Operating Expenses                            
Portfolio Turnover [Heading] Portfolio Turnover                            
Portfolio Turnover [Text Block] The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when fund shares are held in a taxable account. These costs, which are not reflected in annual operating expenses or in the example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 16 % of the average value of its portfolio.                            
Portfolio Turnover, Rate 16.00%                            
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                            
Strategy [Heading] Principal Investment Strategies                            
Strategy Narrative [Text Block] Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal and Massachusetts personal income taxes. The municipal securities in which the fund invests are normally investment-grade (those of medium and high quality). Municipal debt securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Municipal debt securities in which the fund invests include securities issued by U.S. territories and possessions, general obligation bonds of municipalities, local or state governments, project or revenue-specific bonds, or pre-refunded or escrowed bonds, municipal money market securities, and synthetic securities. Municipal securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. Investment grade securities in which the fund invests include securities or issuers rated at least BBB- or its equivalent by at least one nationally recognized statistical ratings organization (NRSRO), or, if unrated, determined by the Adviser to be of comparable quality. Derivative instruments that provide investment exposure to the investments above or exposure to one or more market risk factors associated with such investments are included in the fund's 80% policy, consistent with the fund's investment policies and limitations with respect to investments in derivatives. Managing the fund to have similar overall interest rate risk to the supplemental index (see "Additional Index Information" section of the prospectus). Allocating assets across different market sectors and maturities. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.                            
Bar Chart and Performance Table [Heading] Performance                            
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time. The additional index has characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance. Visit www.fidelity.com for more recent performance information.                            
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time.                            
Performance Availability Website Address [Text] www.fidelity.com                            
Performance Past Does Not Indicate Future [Text] Past performance (before and after taxes) is not an indication of future performance.                            
Bar Chart [Heading] Year-by-Year Returns                            
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                            
Highest Quarterly Return, Date Dec. 31, 2023                            
Highest Quarterly Return 6.91%                            
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                            
Lowest Quarterly Return, Date Mar. 31, 2022                            
Lowest Quarterly Return (6.01%)                            
Performance Table Heading Average Annual Returns                            
Performance Table Uses Highest Federal Rate After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes.                            
Performance Table Explanation after Tax Higher Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.                            
Performance Table Narrative After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.                            
FidelityMassachusettsMunicipalIncomeandMunicipalMoneyMarketFund-RetailComboPRO | Fidelity Massachusetts Municipal Income Fund | After Taxes on Distributions | Fidelity Massachusetts Municipal Income Fund                              
Prospectus Line Items                              
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions                          
Average Annual Return, Percent   4.30%                       0.57% 1.92%
FidelityMassachusettsMunicipalIncomeandMunicipalMoneyMarketFund-RetailComboPRO | Fidelity Massachusetts Municipal Income Fund | After Taxes on Distributions and Sales | Fidelity Massachusetts Municipal Income Fund                              
Prospectus Line Items                              
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions and Sale of Fund Shares                          
Average Annual Return, Percent   3.67%                       1.00% 2.10%
FidelityMassachusettsMunicipalIncomeandMunicipalMoneyMarketFund-RetailComboPRO | Fidelity Massachusetts Municipal Income Fund | IXYHY                              
Prospectus Line Items                              
Average Annual Return, Label [Optional Text]   Bloomberg Massachusetts Enhanced Municipal Linked Index                          
Average Annual Return, Percent   4.14%                       0.52% 2.13%
FidelityMassachusettsMunicipalIncomeandMunicipalMoneyMarketFund-RetailComboPRO | Fidelity Massachusetts Municipal Income Fund | LB015                              
Prospectus Line Items                              
Average Annual Return, Label [Optional Text]   Bloomberg Municipal Bond Index                          
Average Annual Return, Percent   4.25%                       0.80% 2.34%
FidelityMassachusettsMunicipalMoneyMarketFund-PremiumPRO | Fidelity Massachusetts Municipal Money Market Fund | Fidelity Massachusetts Municipal Money Market Fund - Premium Class                              
Prospectus Line Items                              
Management Fees (as a percentage of Assets) 0.20%                            
Distribution and Service (12b-1) Fees 0.00%                            
Other Expenses (as a percentage of Assets): 0.10%                            
Expenses (as a percentage of Assets) 0.30%                            
Expense Example, with Redemption, 1 Year $ 31                            
Expense Example, with Redemption, 3 Years 97                            
Expense Example, with Redemption, 5 Years 169                            
Expense Example, with Redemption, 10 Years $ 381                            
Average Annual Return, Label [Optional Text]   Premium Class                          
Average Annual Return, Percent   2.39%                       1.82% 1.26%
Annual Return [Percent]     2.39% 3.01% 2.85% 0.88% 0.02% 0.37% 1.21% 1.13% 0.56% 0.24%      
FidelityMassachusettsMunicipalMoneyMarketFund-PremiumPRO | Fidelity Massachusetts Municipal Money Market Fund                              
Prospectus Line Items                              
Risk/Return [Heading] Fund Summary Fund /Class: Fidelity® Massachusetts Municipal Money Market Fund /Premium                              
Objective [Heading] Investment Objective                            
Objective, Primary [Text Block] Fidelity® Massachusetts Municipal Money Market Fund seeks as high a level of income, exempt from federal and Massachusetts personal income tax, as is consistent with the preservation of capital and liquidity.                            
Expense Heading [Optional Text] Fee Table                            
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.                            
Shareholder Fees Caption [Optional Text] Shareholder fees                            
Shareholder Fee, Other $ 0                            
Operating Expenses Caption [Optional Text] Annual Operating Expenses                            
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                            
Strategy [Heading] Principal Investment Strategies                            
Strategy Narrative [Text Block] Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal and Massachusetts personal income taxes. Municipal money market securities are high-quality, short-term debt securities that pay a fixed, variable, or floating interest rate, and include variable rate demand notes, commercial paper, and municipal notes. Municipal securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Normally investing in municipal money market securities. Potentially investing up to 20% of assets in municipal securities whose interest is subject to Massachusetts personal income tax. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.                            
Bar Chart and Performance Table [Heading] Performance                            
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year.   Past performance is not an indication of future performance. Visit www.fidelity.com for more recent performance information.                            
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year.                            
Performance Availability Website Address [Text] www.fidelity.com                            
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                            
Bar Chart [Heading] Year-by-Year Returns                            
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                            
Highest Quarterly Return, Date Jun. 30, 2024                            
Highest Quarterly Return 0.80%                            
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                            
Lowest Quarterly Return, Date Mar. 31, 2021                            
Lowest Quarterly Return none                            
Performance Table Heading Average Annual Returns                            
Document Type 485BPOS                            
Registrant Name Fidelity Massachusetts Municipal Trust                            
[1]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.20%   (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect for at least one year from the effective date of the prospectus . FMR may not terminate this arrangement without the approval of the Board of Trustees and may extend it in its discretion after that date.
[2]
A From September 22, 2022 .