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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName FIDELITY MUNICIPAL TRUST
Prospectus Date rr_ProspectusDate Feb. 29, 2020
12.31 Fidelity Flex Municipal Income Fund PRO-05 | Fidelity Flex Municipal Income Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading <p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: bold 12pt Arial, Helvetica, sans-serif;">Fund:<br/>Fidelity Flex℠ Municipal Income Fund</p>
Objective [Heading] rr_ObjectiveHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund seeks to provide a high current yield exempt from federal income tax.
Expense [Heading] rr_ExpenseHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>
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 <p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>
(fees paid directly from your investment) rr_ShareholderFeeOther none
Operating Expenses Caption [Text] rr_OperatingExpensesCaption <p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Portfolio Turnover</b></font>
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. During the most recent fiscal year, the fund's portfolio turnover rate was 18% of the average value of its portfolio.

Portfolio Turnover, Rate rr_PortfolioTurnoverRate 18.00%
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 by, Year, Caption [Text] rr_ExpenseExampleByYearCaption <p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>
Strategy [Heading] rr_StrategyHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>
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 <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>
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 <font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution <font style="FONT-FAMILY: Times New Roman" size="3">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.</font>
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>
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 and compares the performance of the fund's shares to the performance of a securities market index over various periods of time. The index description appears in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance. The performance shown does not reflect the impact of any fees paid at the fee-based account or plan level.

Visit www.401k.com and log in for more recent performance information.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns <font style="FONT-FAMILY: Times New Roman" size="3">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 over various periods of time.</font>
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress <font style="FONT-FAMILY: Times New Roman" size="3">www.401k.com and log in</font>
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture <font style="FONT-FAMILY: Times New Roman" size="3">Past performance (before and after taxes) is not an indication of future performance.</font>
Bar Chart [Heading] rr_BarChartHeading <p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
During the periods shown in the chart:ReturnsQuarter ended
Highest Quarter Return3.06%March 31, 2019
Lowest Quarter Return-1.59%March 31, 2018
Performance Table Heading rr_PerformanceTableHeading <font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate <font style="FONT-FAMILY: Times New Roman" size="3">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.</font>
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher <font style="FONT-FAMILY: Times New Roman" size="3">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.</font>
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock

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.

For the periods ended December 31, 2019
12.31 Fidelity Flex Municipal Income Fund PRO-05 | 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
Total annual operating expenses rr_ExpensesOverAssets none [1]
1 year rr_ExpenseExampleYear01 none
3 years rr_ExpenseExampleYear03 none
5 years rr_ExpenseExampleYear05 none
10 years rr_ExpenseExampleYear10 none
2018 rr_AnnualReturn2018 0.71%
2019 rr_AnnualReturn2019 8.26%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter Return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2019
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 3.06%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter Return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2018
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (1.59%)
Inception Date rr_AverageAnnualReturnInceptionDate Oct. 12, 2017
12.31 Fidelity Flex Municipal Income Fund PRO-05 | Fidelity Flex Municipal Income Fund | Return Before Taxes | Fidelity Flex Municipal Income Fund  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity Flex Municipal Income Fund
Label rr_AverageAnnualReturnLabel <font style="FONT-FAMILY: Times New Roman" ><b>Return Before Taxes</b></font>
Past 1 year rr_AverageAnnualReturnYear01 8.26%
Since Inception rr_AverageAnnualReturnSinceInception 4.35% [2]
12.31 Fidelity Flex Municipal Income Fund PRO-05 | Fidelity Flex Municipal Income Fund | After Taxes on Distributions | Fidelity Flex Municipal Income Fund  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity Flex Municipal Income Fund
Label rr_AverageAnnualReturnLabel <font style="FONT-FAMILY: Times New Roman" ><b>Return After Taxes on Distributions</b></font>
Past 1 year rr_AverageAnnualReturnYear01 8.17%
Since Inception rr_AverageAnnualReturnSinceInception 4.31% [2]
Inception Date rr_AverageAnnualReturnInceptionDate Oct. 12, 2017
12.31 Fidelity Flex Municipal Income Fund PRO-05 | Fidelity Flex Municipal Income Fund | After Taxes on Distributions and Sales | Fidelity Flex Municipal Income Fund  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity Flex Municipal Income Fund
Label rr_AverageAnnualReturnLabel <font style="FONT-FAMILY: Times New Roman" ><b>Return After Taxes on Distributions and Sale of Fund Shares</b></font>
Past 1 year rr_AverageAnnualReturnYear01 6.09%
Since Inception rr_AverageAnnualReturnSinceInception 3.93% [2]
Inception Date rr_AverageAnnualReturnInceptionDate Oct. 12, 2017
12.31 Fidelity Flex Municipal Income Fund PRO-05 | Fidelity Flex Municipal Income Fund | Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 7.54%
Since Inception rr_AverageAnnualReturnSinceInception 4.13% [2]
[1] 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.
[2] From October 12, 2017