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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName FIDELITY MASSACHUSETTS MUNICIPAL TRUST
Prospectus Date rr_ProspectusDate Mar. 31, 2018
01.31 Fidelity Massachusetts AMT Tax-Free Money Market Fund Service PRO-04 | Fidelity® Massachusetts AMT Tax-Free Money Market Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Massachusetts AMT Tax-Free Money Market Fund/Service
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The 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] rr_ExpenseHeading Fee Table
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption

Shareholder fees
(fees paid directly from your investment) rr_ShareholderFeeOther none
Operating Expenses Caption [Text] rr_OperatingExpensesCaption

Annual Operating Expenses

(expenses that you pay each year as a % of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination will remain in effect for at least one year from the effective date of the prospectus
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock

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 your shareholder 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:

Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption

Sell All Shares
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
  • Normally investing in municipal money market securities.
  • Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal and Massachusetts personal income taxes.
  • Potentially investing up to 20% of assets in municipal securities whose interest is subject to Massachusetts personal income tax.
  • Normally not investing in municipal securities whose interest is subject to the federal alternative minimum 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.
Risk [Heading] rr_RiskHeading Principal Investment Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock
  • Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.
  • Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.
  • Foreign Exposure. Entities providing credit support or a maturity-shortening structure that are located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.
  • Geographic Concentration. Unfavorable political or economic conditions within Massachusetts can affect the credit quality of issuers located in that state.
  • Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text] rr_RiskLoseMoney You could lose money by investing in the fund.
Risk Money Market Fund [Text] rr_RiskMoneyMarketFund Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

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 institutional.fidelity.com for more recent performance information.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The information illustrates the changes in the performance of the fund's shares from year to year.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress institutional.fidelity.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture Past performance is not an indication of future performance.
Bar Chart [Heading] rr_BarChartHeading

Year-by-Year Returns

Calendar Years
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
During the periods shown in the chart:ReturnsQuarter ended
Highest Quarter Return0.54%March 31, 2008
Lowest Quarter Return0.00%March 31, 2015
Performance Table Heading rr_PerformanceTableHeading Average Annual Returns
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2017
01.31 Fidelity Massachusetts AMT Tax-Free Money Market Fund Service PRO-04 | Fidelity® Massachusetts AMT Tax-Free Money Market Fund | Fidelity Massachusetts AMT Tax-Free Money Market Fund-Service Class AMT  
Risk/Return: rr_RiskReturnAbstract  
Management fee rr_ManagementFeesOverAssets 0.20%
Distribution and/or Service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses rr_OtherExpensesOverAssets 0.05%
Total annual operating expenses rr_ExpensesOverAssets 0.50%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.05% [1]
Total annual operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.45%
1 year rr_ExpenseExampleYear01 $ 46
3 years rr_ExpenseExampleYear03 144
5 years rr_ExpenseExampleYear05 252
10 years rr_ExpenseExampleYear10 $ 567
2008 rr_AnnualReturn2008 1.70%
2009 rr_AnnualReturn2009 0.09%
2010 rr_AnnualReturn2010 0.01%
2011 rr_AnnualReturn2011 0.01%
2012 rr_AnnualReturn2012 0.02%
2013 rr_AnnualReturn2013 0.02%
2014 rr_AnnualReturn2014 0.04%
2015 rr_AnnualReturn2015 0.06%
2016 rr_AnnualReturn2016 0.14%
2017 rr_AnnualReturn2017 0.41%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter Return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2008
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 0.54%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter Return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2015
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn none
01.31 Fidelity Massachusetts AMT Tax-Free Money Market Fund Service PRO-04 | Fidelity® Massachusetts AMT Tax-Free Money Market Fund | Return Before Taxes | Fidelity Massachusetts AMT Tax-Free Money Market Fund-Service Class AMT  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity® Massachusetts AMT Tax-Free Money Market Fund
Label rr_AverageAnnualReturnLabel Service Class
Past 1 year rr_AverageAnnualReturnYear01 0.41%
Past 5 years rr_AverageAnnualReturnYear05 0.13%
Past 10 years rr_AverageAnnualReturnYear10 0.25%
[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Service Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, certain securities lending costs, brokerage commissions, fees and expenses of the Independent Trustees, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of its average net assets, exceed 0.45%. 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.