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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName FIDELITY MUNICIPAL TRUST
Prospectus Date rr_ProspectusDate Mar. 01, 2019
12.31 Fidelity Conservative Income Municipal Bond Fund Retail PRO-07 | Fidelity Conservative Income Municipal Bond 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: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Fidelity® Conservative Income Municipal Bond Fund</b>/Fidelity® Conservative Income Municipal Bond 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 level of income, exempt from federal income tax, consistent with preservation of capital.
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>
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination <font style="FONT-FAMILY: Times New Roman" size="3">April 30, 2020</font>
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 45% of the average value of its portfolio.

Portfolio Turnover, Rate rr_PortfolioTurnoverRate 45.00%
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. 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 its assets in U.S. dollar-denominated municipal money market securities and high quality investment-grade municipal debt securities whose interest is exempt from federal income tax.
  • Investing up to 10% of assets in lower quality investment-grade securities.
  • Managing the fund to have similar overall interest rate risk to the 50% Bloomberg Barclays Municipal Bond 1 Year (1-2 Y) Index / 50% iMoneyNet℠ All Tax-Free National Retail Money Market Funds Average.
  • Normally maintaining a dollar-weighted average maturity of one year or less.
  • Normally investing in securities with a maximum maturity of four years.
  • 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 or money market 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 and a hypothetical composite of market indexes over various periods of time. The indexes have 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] 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 and a hypothetical composite of market indexes over various periods of time.</font>
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress <font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com</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 Return0.51%December 31, 2018
Lowest Quarter Return-0.30%December 31, 2016
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 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.

For the periods ended December 31, 2018
12.31 Fidelity Conservative Income Municipal Bond Fund Retail PRO-07 | Fidelity Conservative Income Municipal Bond Fund | Fidelity Conservative Income Municipal Bond Fund  
Risk/Return: rr_RiskReturnAbstract  
Management fee rr_ManagementFeesOverAssets 0.30%
Distribution and/or Service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 0.10%
Total annual operating expenses rr_ExpensesOverAssets 0.40%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.05% [1]
Total annual operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.35%
1 year rr_ExpenseExampleYear01 $ 36
3 years rr_ExpenseExampleYear03 122
5 years rr_ExpenseExampleYear05 218
10 years rr_ExpenseExampleYear10 $ 499
2014 rr_AnnualReturn2014 0.36%
2015 rr_AnnualReturn2015 0.32%
2016 rr_AnnualReturn2016 0.21%
2017 rr_AnnualReturn2017 1.00%
2018 rr_AnnualReturn2018 1.45%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter Return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2018
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 0.51%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter Return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2016
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (0.30%)
Inception Date rr_AverageAnnualReturnInceptionDate Oct. 15, 2013
12.31 Fidelity Conservative Income Municipal Bond Fund Retail PRO-07 | Fidelity Conservative Income Municipal Bond Fund | Return Before Taxes | Fidelity Conservative Income Municipal Bond Fund  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity Conservative Income Municipal Bond Fund
Label rr_AverageAnnualReturnLabel <font style="FONT-FAMILY: Times New Roman" ><b>Return Before Taxes</b></font>
Past 1 year rr_AverageAnnualReturnYear01 1.45%
Past 5 years rr_AverageAnnualReturnYear05 0.67%
Since Inception rr_AverageAnnualReturnSinceInception 0.68% [2]
12.31 Fidelity Conservative Income Municipal Bond Fund Retail PRO-07 | Fidelity Conservative Income Municipal Bond Fund | After Taxes on Distributions | Fidelity Conservative Income Municipal Bond Fund  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity Conservative Income Municipal Bond Fund
Label rr_AverageAnnualReturnLabel <font style="FONT-FAMILY: Times New Roman" ><b>Return After Taxes on Distributions</b></font>
Past 1 year rr_AverageAnnualReturnYear01 1.45%
Past 5 years rr_AverageAnnualReturnYear05 0.66%
Since Inception rr_AverageAnnualReturnSinceInception 0.68% [2]
Inception Date rr_AverageAnnualReturnInceptionDate Oct. 15, 2013
12.31 Fidelity Conservative Income Municipal Bond Fund Retail PRO-07 | Fidelity Conservative Income Municipal Bond Fund | After Taxes on Distributions and Sales | Fidelity Conservative Income Municipal Bond Fund  
Risk/Return: rr_RiskReturnAbstract  
Column rr_AverageAnnualReturnColumnName Fidelity Conservative Income Municipal Bond 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 1.41%
Past 5 years rr_AverageAnnualReturnYear05 0.66%
Since Inception rr_AverageAnnualReturnSinceInception 0.67% [2]
Inception Date rr_AverageAnnualReturnInceptionDate Oct. 15, 2013
12.31 Fidelity Conservative Income Municipal Bond Fund Retail PRO-07 | Fidelity Conservative Income Municipal Bond Fund | Bloomberg Barclays Municipal Bond 1 Year (1-2 Y) Index(reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 1.74%
Past 5 years rr_AverageAnnualReturnYear05 0.83%
Since Inception rr_AverageAnnualReturnSinceInception 0.83% [2]
12.31 Fidelity Conservative Income Municipal Bond Fund Retail PRO-07 | Fidelity Conservative Income Municipal Bond Fund | Fidelity Conservative Income Municipal Bond Composite Index℠(reflects no deduction for fees, expenses, or taxes)  
Risk/Return: rr_RiskReturnAbstract  
Past 1 year rr_AverageAnnualReturnYear01 1.60%
Past 5 years rr_AverageAnnualReturnYear05 0.72%
Since Inception rr_AverageAnnualReturnSinceInception 0.70% [2]
[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse the class of shares 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, 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 0.35% (the Expense Cap). If at any time during the current fiscal year expenses for the class of shares 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 fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through April 30, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.
[2] From October 15, 2013