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Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName BLACKROCK MUNICIPAL SERIES TRUST
Prospectus Date rr_ProspectusDate Sep. 28, 2012
Investor and Institutional Shares | BlackRock Intermediate Municipal Fund of BlackRock Municipal Series Trust
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Fund Overview

Key Facts about BlackRock Intermediate Municipal Fund
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock The investment objective of the BlackRock Intermediate Municipal Fund (the “Fund”) is to provide shareholders with a high level of income exempt from Federal income taxes.
Expense [Heading] rr_ExpenseHeading Fees and Expenses of the Fund
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock This table describes the fees and expenses that you may pay if you buy and hold shares of the Fund. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in the BlackRock-advised fund complex. More information about these and other discounts is available from your financial professional and in the “Details about the Share Classes” section on page 16 of the Fund’s prospectus and in the “Purchase of Shares” section on page II-69 of the Fund’s statement of additional information.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption Shareholder Fees
(fees paid directly from your investment)
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses
(expenses that you pay each year as a
percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination October 1, 2013
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 may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 15% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 15.00%
Expense Breakpoint Discounts [Text] rr_ExpenseBreakpointDiscounts You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in the BlackRock-advised fund complex.
Expense Breakpoint, Minimum Investment Required [Amount] rr_ExpenseBreakpointMinimumInvestmentRequiredAmount 100,000
Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text] rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees The Total Annual Fund Operating Expenses and the Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements do not correlate to the ratio of expenses to average net assets given in the Fund’s annual report, which does not include Acquired Fund Fees and Expenses.
Expense Example [Heading] rr_ExpenseExampleHeading Example:
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The Example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example, No Redemption Narrative [Text Block] rr_ExpenseExampleNoRedemptionNarrativeTextBlock You will pay the following expenses if you do not redeem your shares:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies of the Fund
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock The Fund invests primarily in a portfolio of investment grade municipal bonds. These may be obligations of a variety of issuers including states, territories and possessions of the United States, including the District of Columbia, and their political subdivisions, agencies and instrumentalities. Under normal circumstances, the Fund will invest at least 80% of its assets in municipal bonds. The Fund may invest in either fixed rate or variable rate obligations. Under normal circumstances, at least 80% of the Fund’s assets will be invested in investment grade municipal bonds. Under normal circumstances, the Fund’s dollar-weighted average maturity will be between three and ten years; however, there is no limit on the remaining maturity of each individual municipal bond investment by the Fund.
Risk [Heading] rr_RiskHeading Principal Risks of Investing in the Fund
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock Risk is inherent in all investing. The value of your investment in the Fund, as well as the amount of return you receive on your investment, may fluctuate significantly from day to day and over time. You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments. The following is a summary description of principal risks of investing in the Fund.
  • Credit Risk — Credit risk refers to the possibility that the issuer of a security will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer.
  • Prepayment and Extension Risk — When interest rates fall, an issuer may redeem a security with call features by repaying it early, and the Fund may have to invest the proceeds in securities with lower yields. When interest rates rise, certain obligations will be paid off by the issuer more slowly than anticipated, causing the value of these securities to fall.
  • Interest Rate Risk — Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall, and decrease as interest rates rise.
  • Liquidity Risk — Liquidity risk exists when particular investments are difficult to purchase or sell. The Fund’s investments in illiquid securities may reduce the returns of the Fund because it may be difficult to sell the illiquid securities at an advantageous time or price. To the extent that the Fund’s principal investment strategies involve derivatives or securities with substantial market and/or credit risk, the Fund will tend to have the greatest exposure to liquidity risk. Liquid investments may become illiquid after purchase by the Fund, particularly during periods of market turmoil. Illiquid investments may be harder to value, especially in changing markets, and if the Fund is forced to sell these investments to meet redemption requests or for other cash needs, the Fund may suffer a loss. In addition, when there is illiquidity in the market for certain securities, the Fund, due to limitations on illiquid investments, may be subject to purchase and sale restrictions.
  • Market Risk and Selection Risk — Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money.
  • Municipal Securities Risks — Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:

    General Obligation Bonds Risks — Timely payments depend on the issuer’s credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.

    Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.

    Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its faith, credit and taxing power for repayment.

    Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.

    Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.

    Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.

    Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers’ bond counsel and, in the case of derivative securities, sponsors’ counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities.
Risk Lose Money [Text] rr_RiskLoseMoney You may lose part or all of your investment in the Fund or your investment may not perform as well as other similar investments.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance Information
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund. The table compares the Fund’s performance to those of the S&P Municipal Bond Index and the S&P Intermediate Municipal Bond Index, which are relevant to the Fund because they have characteristics similar to the Fund’s investment strategies. Prior to the Investor A and Investor C Shares inception date of October 2, 2006, the returns for Investor A and Investor C Shares are based on performance of the Fund’s Institutional Shares. The returns for Investor A and Investor C Shares, however, are adjusted to reflect the distribution and service (12b-1) fees applicable to such Shares. As with all such investments, past performance (before and after taxes) is not an indication of future results. Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown. However, the table includes all applicable fees and sales charges. If the Fund’s investment manager and its affiliates had not waived or reimbursed certain Fund expenses during these periods, the Fund’s returns would have been lower. Updated information on the Fund’s performance can be obtained by visiting http://www.blackrock.com/funds or can be obtained by phone at 800-882-0052.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The information shows you how the Fund’s performance has varied year by year and provides some indication of the risks of investing in the Fund.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 800-882-0052
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress http://www.blackrock.com/funds
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture As with all such investments, past performance (before and after taxes) is not an indication of future results.
Bar Chart [Heading] rr_BarChartHeading
Institutional Shares
ANNUAL TOTAL RETURNS
BlackRock Intermediate Municipal Fund
As of 12/31
Bar Chart Does Not Reflect Sales Loads [Text] rr_BarChartDoesNotReflectSalesLoads Sales charges are not reflected in the bar chart. If they were, returns would be less than those shown.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock During the ten-year period shown in the bar chart, the highest return for a quarter was 7.08% (quarter ended September 30, 2009) and the lowest return for a quarter was –4.09% (quarter ended December 31, 2010). The year-to-date return as of June 30, 2012 was 4.32%.
Performance Table Heading rr_PerformanceTableHeading As of 12/31/11
Average Annual Total Returns
Performance Table Does Reflect Sales Loads rr_PerformanceTableDoesReflectSalesLoads However, the table includes all applicable fees and sales charges.
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred Actual after-tax returns depend on the investor's tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements such as 401(k) plans or individual retirement accounts.
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A, Investor B and Investor C Shares will vary.
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock After-tax returns are calculated using the historical highest individual Federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on the investor's tax situation and may differ from those shown, and the after-tax returns shown are not relevant to investors who hold their shares through tax-deferred arrangements such as 401(k) plans or individual retirement accounts. After-tax returns are shown for Institutional Shares only, and the after-tax returns for Investor A, Investor B and Investor C Shares will vary.
Investor and Institutional Shares | BlackRock Intermediate Municipal Fund of BlackRock Municipal Series Trust | Investor A Shares
  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice 4.25%
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower) rr_MaximumDeferredSalesChargeOverOfferingPrice none [1]
Management Fee rr_ManagementFeesOverAssets 0.55% [2]
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Interest Expense rr_Component1OtherExpensesOverAssets 0.01%
Miscellaneous Other Expense rr_Component2OtherExpensesOverAssets 0.13%
Other Expenses rr_OtherExpensesOverAssets 0.14%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01% [3]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.95% [3]
Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.05%) [2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 0.90% [2],[3]
1 Year rr_ExpenseExampleYear01 513
3 Years rr_ExpenseExampleYear03 710
5 Years rr_ExpenseExampleYear05 923
10 Years rr_ExpenseExampleYear10 1,537
1 Year rr_AverageAnnualReturnYear01 6.34%
5 Years rr_AverageAnnualReturnYear05 4.21%
10 Years rr_AverageAnnualReturnYear10 4.31%
Investor and Institutional Shares | BlackRock Intermediate Municipal Fund of BlackRock Municipal Series Trust | Investor B Shares
  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower) rr_MaximumDeferredSalesChargeOverOfferingPrice 1.00% [4]
Management Fee rr_ManagementFeesOverAssets 0.55% [2]
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.30%
Interest Expense rr_Component1OtherExpensesOverAssets 0.01%
Miscellaneous Other Expense rr_Component2OtherExpensesOverAssets 0.21%
Other Expenses rr_OtherExpensesOverAssets 0.22%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01% [3]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.08% [3]
Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.05%) [2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 1.03% [2],[3]
1 Year rr_ExpenseExampleYear01 205
3 Years rr_ExpenseExampleYear03 364
5 Years rr_ExpenseExampleYear05 591
10 Years rr_ExpenseExampleYear10 1,313
1 Year rr_ExpenseExampleNoRedemptionYear01 105
3 Years rr_ExpenseExampleNoRedemptionYear03 339
5 Years rr_ExpenseExampleNoRedemptionYear05 591
10 Years rr_ExpenseExampleNoRedemptionYear10 1,313
1 Year rr_AverageAnnualReturnYear01 10.02%
5 Years rr_AverageAnnualReturnYear05 5.02%
10 Years rr_AverageAnnualReturnYear10 4.69%
Investor and Institutional Shares | BlackRock Intermediate Municipal Fund of BlackRock Municipal Series Trust | Investor C Shares
  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower) rr_MaximumDeferredSalesChargeOverOfferingPrice 1.00% [5]
Management Fee rr_ManagementFeesOverAssets 0.55% [2]
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 1.00%
Interest Expense rr_Component1OtherExpensesOverAssets 0.01%
Miscellaneous Other Expense rr_Component2OtherExpensesOverAssets 0.15%
Other Expenses rr_OtherExpensesOverAssets 0.16%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01% [3]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.72% [3]
Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.05%) [2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 1.67% [2],[3]
1 Year rr_ExpenseExampleYear01 270
3 Years rr_ExpenseExampleYear03 537
5 Years rr_ExpenseExampleYear05 929
10 Years rr_ExpenseExampleYear10 2,026
1 Year rr_ExpenseExampleNoRedemptionYear01 170
3 Years rr_ExpenseExampleNoRedemptionYear03 537
5 Years rr_ExpenseExampleNoRedemptionYear05 929
10 Years rr_ExpenseExampleNoRedemptionYear10 2,026
1 Year rr_AverageAnnualReturnYear01 9.22%
5 Years rr_AverageAnnualReturnYear05 4.33%
10 Years rr_AverageAnnualReturnYear10 3.98%
Investor and Institutional Shares | BlackRock Intermediate Municipal Fund of BlackRock Municipal Series Trust | Institutional Shares
  
Risk/Return: rr_RiskReturnAbstract  
Maximum Sales Charge (Load) Imposed on Purchases (as percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as percentage of offering price or redemption proceeds, whichever is lower) rr_MaximumDeferredSalesChargeOverOfferingPrice none
Management Fee rr_ManagementFeesOverAssets 0.55% [2]
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Interest Expense rr_Component1OtherExpensesOverAssets 0.01%
Miscellaneous Other Expense rr_Component2OtherExpensesOverAssets 0.14%
Other Expenses rr_OtherExpensesOverAssets 0.15%
Acquired Fund Fees and Expenses rr_AcquiredFundFeesAndExpensesOverAssets 0.01% [3]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.71% [3]
Fee Waivers and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.05%) [2]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements rr_NetExpensesOverAssets 0.66% [2],[3]
1 Year rr_ExpenseExampleYear01 67
3 Years rr_ExpenseExampleYear03 222
5 Years rr_ExpenseExampleYear05 390
10 Years rr_ExpenseExampleYear10 878
2002 rr_AnnualReturn2002 10.41%
2003 rr_AnnualReturn2003 4.90%
2004 rr_AnnualReturn2004 3.04%
2005 rr_AnnualReturn2005 1.23%
2006 rr_AnnualReturn2006 4.04%
2007 rr_AnnualReturn2007 3.25%
2008 rr_AnnualReturn2008 (4.00%)
2009 rr_AnnualReturn2009 13.38%
2010 rr_AnnualReturn2010 3.82%
2011 rr_AnnualReturn2011 11.32%
Year to Date Return, Label rr_YearToDateReturnLabel year-to-date return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Jun. 30, 2012
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 4.32%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Sep. 30, 2009
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 7.08%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2010
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (4.09%)
1 Year rr_AverageAnnualReturnYear01 11.32%
5 Years rr_AverageAnnualReturnYear05 5.37%
10 Years rr_AverageAnnualReturnYear10 5.02%
Investor and Institutional Shares | BlackRock Intermediate Municipal Fund of BlackRock Municipal Series Trust | Return After Taxes on Distributions | Institutional Shares
  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 11.32%
5 Years rr_AverageAnnualReturnYear05 5.37%
10 Years rr_AverageAnnualReturnYear10 4.94%
Investor and Institutional Shares | BlackRock Intermediate Municipal Fund of BlackRock Municipal Series Trust | Return After Taxes on Distributions and Sale of Fund Shares | Institutional Shares
  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 8.74%
5 Years rr_AverageAnnualReturnYear05 5.13%
10 Years rr_AverageAnnualReturnYear10 4.83%
Investor and Institutional Shares | BlackRock Intermediate Municipal Fund of BlackRock Municipal Series Trust | S&P Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 10.62%
5 Years rr_AverageAnnualReturnYear05 4.86%
10 Years rr_AverageAnnualReturnYear10 5.35%
Investor and Institutional Shares | BlackRock Intermediate Municipal Fund of BlackRock Municipal Series Trust | S&P Intermediate Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
  
Risk/Return: rr_RiskReturnAbstract  
1 Year rr_AverageAnnualReturnYear01 10.17%
5 Years rr_AverageAnnualReturnYear05 5.96%
10 Years rr_AverageAnnualReturnYear10 5.57%
[1] A contingent deferred sales charge ("CDSC") of 1.00% is assessed on certain redemptions of Investor A Shares made within 18 months after purchase where no initial sales charge was paid at time of purchase as part of an investment of $1,000,000 or more.
[2] As described in the "Management of the Fund" section of the Fund's prospectus on page 32, BlackRock Advisors, LLC has contractually agreed to waive 0.05% of its management fee until October 1, 2013. The agreement may be terminated upon 90 days' notice by a majority of the non-interested trustees of the Fund or by a vote of a majority of the outstanding voting securities of the Fund.
[3] The Total Annual Fund Operating Expenses and the Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements do not correlate to the ratio of expenses to average net assets given in the Fund's annual report, which does not include Acquired Fund Fees and Expenses.
[4] The CDSC is 1.00% if shares are redeemed in less than one year. The CDSC for Investor B Shares decreases for redemptions made in subsequent years. After three years there is no CDSC on Investor B Shares. (See the section "Details about the Share Classes - Investor B Shares" for the complete schedule of CDSCs.)
[5] There is no CDSC on Investor C Shares after one year.