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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName FIDELITY MUNICIPAL TRUST
Prospectus Date rr_ProspectusDate Oct. 11, 2017
NF_Fidelity Flex Municipal Income Fund_PRO-01 | Fidelity Flex Municipal Income Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading

Fund Summary

Fund:
Fidelity Flex℠ Municipal Income Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund seeks to provide a high current yield exempt from federal income tax.
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)
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock

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.

Other Expenses, New Fund, Based on Estimates [Text] rr_OtherExpensesNewFundBasedOnEstimates Based on estimated amounts for the current fiscal year.
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. This example does not include any fees paid at the fee-based account or plan level. 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, No Redemption, By Year, Caption [Text] rr_ExpenseExampleNoRedemptionByYearCaption

Hold Shares
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
  • Normally investing at least 80% of assets in investment-grade municipal securities whose interest is exempt from federal income tax.
  • Managing the fund to have similar overall interest rate risk to the Bloomberg Barclays Municipal Bond Index.
  • 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.
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 debt security to decrease.
  • Issuer-Specific Changes. The value of an individual security or particular type of security can be more volatile than, and can perform differently from, the market as a whole. 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 security to decrease.

An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. You could lose money by investing in the fund.

Risk Lose Money [Text] rr_RiskLoseMoney You could lose money by investing in the fund.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

Performance history will be available for the fund after the fund has been in operation for one calendar year.
Performance One Year or Less [Text] rr_PerformanceOneYearOrLess Performance history will be available for the fund after the fund has been in operation for one calendar year.
NF_Fidelity Flex Municipal Income Fund_PRO-01 | Fidelity Flex Municipal Income Fund | Fidelity Flex Municipal Income Fund  
Risk/Return: rr_RiskReturnAbstract  
Management fee rr_ManagementFeesOverAssets none
Distribution and/or Service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets none [1]
Total annual operating expenses rr_ExpensesOverAssets none [2]
1 Year rr_ExpenseExampleNoRedemptionYear01 none
3 Years rr_ExpenseExampleNoRedemptionYear03 none
[1] Based on estimated amounts for the current fiscal year.
[2] The fund is available only to certain fee-based accounts offered by Fidelity. Managed account clients, retirement plans, plan sponsors and/or plan participants pay a wrap fee that covers investment advisory and administrative services.