Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	BLACKROCK MULTI STATE MUNICIPAL SERIES TRUST
Prospectus Date	rr_ProspectusDate	Sep. 28, 2017
Service | BlackRock Pennsylvania Municipal Bond Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Overview Key Facts About BlackRock Pennsylvania Municipal Bond Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The investment objective of BlackRock Pennsylvania Municipal Bond Fund (the “Pennsylvania Fund” or the “Fund”) is to provide shareholders with income exempt from Federal income tax and Pennsylvania personal 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.
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	September 30, 2018
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 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 18% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	18.00%
Expenses Restated to Reflect Current [Text]	rr_ExpensesRestatedToReflectCurrent	Management Fee has been restated to reflect current fees.
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:
Strategy [Heading]	rr_StrategyHeading	Principal Investment Strategies of the Fund
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	The Fund invests primarily in a portfolio of long-term investment grade Pennsylvania municipal bonds. Pennsylvania municipal bonds are debt obligations issued by or on behalf of a governmental entity in Pennsylvania or other qualifying issuer that pay interest that, in the opinion of bond counsel, is excludable from gross income for Federal income tax purposes (except that interest may be includable in taxable income for purposes of the Federal alternative minimum tax) and exempt from Pennsylvania personal income taxes. These may be obligations of a variety of issuers including governmental entities in Pennsylvania and issuers located in Puerto Rico, the U.S. Virgin Islands and Guam. Under normal circumstances, the Fund will invest at least 80% of its assets in Pennsylvania municipal bonds. At least 80% of the Fund’s assets will be invested in investment grade securities. Investment grade securities are securities which are rated in the four highest categories by at least one of the major rating agencies or determined by the management team to be of similar quality. When choosing investments, Fund management considers various factors, including the credit quality of issuers, yield analysis, maturity analysis and the call features of the obligations. Under normal circumstances, the Fund’s weighted average maturity will be more than ten years. The Fund may leverage its assets through the use of proceeds received through tender option bond transactions. In a tender option bond transaction, the Fund transfers municipal bonds or other municipal securities into a special purpose entity (a “TOB Trust”). A TOB Trust typically issues two classes of beneficial interests: short-term floating rate interests (“TOB Floaters”), which are sold to third party investors, and residual inverse floating rate interests (“TOB Residuals”), which are generally issued to the Fund. The Fund may invest in TOB Residuals and may also invest in TOB Floaters. The Fund will look through to the underlying municipal bond held by a TOB Trust for purposes of the Fund’s 80% policy.
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. Debt Securities Risk — Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things. Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. 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. The Fund may be subject to a greater risk of rising interest rates due to the current period of historically low rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance. Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make principal and interest payments 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. The degree of credit risk depends on the issuer’s financial condition and on the terms of the securities. Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields. 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 a 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. Non-Diversification Risk — The Fund is a non-diversified fund. Because the Fund may invest in securities of a smaller number of issuers, it may be more exposed to the risks associated with and developments affecting an individual issuer than a fund that invests more widely. State Specific Risk — The Fund will invest primarily in municipal securities issued by or on behalf of the Commonwealth of Pennsylvania. As a result, the Fund is more exposed to risks affecting issuers of Pennsylvania municipal securities than is a municipal securities fund that invests more widely. Tender Option Bonds Risk — The Fund’s participation in tender option bond transactions may reduce the Fund’s returns and/or increase volatility. Investments in tender option bond transactions expose the Fund to counterparty risk and leverage risk. An investment in a tender option bond transaction typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on TOB Residuals will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals generally will underperform the market for fixed rate municipal securities in a rising interest rate environment. The Fund may invest in TOB Trusts on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis, it could suffer losses in excess of the value of its TOB Residuals.
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.
Risk Nondiversified Status [Text]	rr_RiskNondiversifiedStatus	Non-Diversification Risk — The Fund is a non-diversified fund. Because the Fund may invest in securities of a smaller number of issuers, it may be more exposed to the risks associated with and developments affecting an individual issuer than a fund that invests more widely.
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 that of the S&P® Municipal Bond Index and the Custom Pennsylvania Index. To the extent that dividends and distributions have been paid by the Fund, the performance information for the Fund in the chart and table assumes reinvestment of the dividends and distributions. As with all such investments, past performance (before and after taxes) is not an indication of future results. The table includes all applicable fees. 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, including its current net asset value, can be obtained by visiting http://www.blackrock.com 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
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	Service Shares ANNUAL TOTAL RETURNS BlackRock Pennsylvania Municipal Bond Fund As of 12/31
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	During the ten-year period shown in the bar chart, the highest return for a quarter was 8.60% (quarter ended September 30, 2009) and the lowest return for a quarter was -6.63% (quarter ended December 31, 2010). The year-to-date return as of June 30, 2017 was 3.57%.
Performance Table Heading	rr_PerformanceTableHeading	As of 12/31/16 Average Annual Total Returns
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 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.
Service | BlackRock Pennsylvania Municipal Bond Fund | Service Shares
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.52%	[1],[2]
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Interest Expense	rr_Component1OtherExpensesOverAssets	0.17%
Miscellaneous Other Expenses	rr_Component2OtherExpensesOverAssets	0.16%
Other Expenses	rr_OtherExpensesOverAssets	0.33%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	1.10%	[2],[3]
Fee Waivers and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.13%)	[1],[4]
Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.97%	[1],[4]
1 Year	rr_ExpenseExampleYear01	$ 99
3 Years	rr_ExpenseExampleYear03	337
5 Years	rr_ExpenseExampleYear05	594
10 Years	rr_ExpenseExampleYear10	$ 1,329
2007	rr_AnnualReturn2007	1.54%
2008	rr_AnnualReturn2008	(7.72%)
2009	rr_AnnualReturn2009	15.96%
2010	rr_AnnualReturn2010	0.47%
2011	rr_AnnualReturn2011	13.43%
2012	rr_AnnualReturn2012	9.08%
2013	rr_AnnualReturn2013	(5.75%)
2014	rr_AnnualReturn2014	12.41%
2015	rr_AnnualReturn2015	3.82%
2016	rr_AnnualReturn2016	0.01%
Year to Date Return, Label	rr_YearToDateReturnLabel	year-to-date return
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Jun. 30, 2017
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	3.57%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	8.60%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2010
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(6.63%)
1 Year	rr_AverageAnnualReturnYear01	0.01%
5 Years	rr_AverageAnnualReturnYear05	3.71%
10 Years	rr_AverageAnnualReturnYear10	4.04%
Service | BlackRock Pennsylvania Municipal Bond Fund | Return After Taxes on Distributions | Service Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	0.01%
5 Years	rr_AverageAnnualReturnYear05	3.71%
10 Years	rr_AverageAnnualReturnYear10	4.02%
Service | BlackRock Pennsylvania Municipal Bond Fund | Return After Taxes on Distributions and Sale of Fund Shares | Service Shares
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	1.67%
5 Years	rr_AverageAnnualReturnYear05	3.81%
10 Years	rr_AverageAnnualReturnYear10	4.07%
Service | BlackRock Pennsylvania Municipal Bond Fund | S&P ® Municipal Bond Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	0.77%
5 Years	rr_AverageAnnualReturnYear05	3.55%
10 Years	rr_AverageAnnualReturnYear10	4.20%
Service | BlackRock Pennsylvania Municipal Bond Fund | Custom Pennsylvania Index (Reflects no deduction for fees, expenses or taxes)
Risk/Return:	rr_RiskReturnAbstract
1 Year	rr_AverageAnnualReturnYear01	0.83%	[5]
5 Years	rr_AverageAnnualReturnYear05	4.22%	[5]
10 Years	rr_AverageAnnualReturnYear10	4.62%	[5]

[1] As described in the “Management of the Funds” section of the Fund’s prospectus beginning on page 32, BlackRock Advisors, LLC (“BlackRock”) has contractually agreed to waive the management fee with respect to any portion of the Fund’s assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BlackRock or its affiliates through September 30, 2018. The contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of BlackRock Multi-State Municipal Series Trust or by a vote of a majority of the outstanding voting securities of the Fund.

[2] Management Fee has been restated to reflect current fees.

[3] The Total Annual Fund Operating Expenses do not correlate to the ratios of expenses to average net assets given in the Fund’s most recent annual report, which do not include the restatement of Management Fees to reflect current fees.

[4] As described in the “Management of the Funds” section of the Fund’s prospectus, BlackRock has contractually agreed to waive and/or reimburse fees and/or expenses in order to limit Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements (excluding Interest Expense, Dividend Expense, Acquired Fund Fees and Expenses and certain other Fund expenses) as a percentage of average daily net assets to 0.80% for Service Shares through September 30, 2018. This contractual agreement may be terminated upon 90 days’ notice by a majority of the non-interested trustees of BlackRock Multi-State Municipal Series Trust or by a vote of a majority of the outstanding voting securities of the Fund.

[5] The Custom Pennsylvania Index reflects the returns of the S&P® Pennsylvania Municipal Bond Index for periods prior to January 1, 2013, and the returns of only those Pennsylvania bonds in the S&P® Pennsylvania Municipal Bond Index that have maturities greater than 5 years for periods subsequent to January 1, 2013.