<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>30</ContextCount>
  <ElementCount>183</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>16</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report instance="d51288d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001 - Document - Document and Entity Information - {Elements}</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DocumentAndEntityInformationElements</Role>
      <ShortName>Document and Entity Information -</ShortName>
      <MenuCategory></MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="d51288d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0002 - Document - Risk/Return Summary {Unlabeled} - BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Investor A, C and Institutional)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/RiskReturnSummaryUnlabeledBlackrockStrategicMunicipalOpportunitiesFundOfBlackrockMunicipalSeriesTrustInvestorACAndInstitutional</Role>
      <ShortName>Risk/Return Summary- BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Investor A, C and Institutional)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="d51288d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>0009 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Investor A, C and Institutional)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/RiskReturnDetailDataElementsBlackrockStrategicMunicipalOpportunitiesFundOfBlackrockMunicipalSeriesTrustInvestorACAndInstitutional</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Investor A, C and Institutional)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="d51288d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>0010 - Document - Risk/Return Summary {Unlabeled} - BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Investor A1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/RiskReturnSummaryUnlabeledBlackrockStrategicMunicipalOpportunitiesFundOfBlackrockMunicipalSeriesTrustInvestorA1</Role>
      <ShortName>Risk/Return Summary- BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Investor A1)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="d51288d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>0016 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Investor A1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/RiskReturnDetailDataElementsBlackrockStrategicMunicipalOpportunitiesFundOfBlackrockMunicipalSeriesTrustInvestorA1</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Investor A1)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="d51288d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>0017 - Document - Risk/Return Summary {Unlabeled} - BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Class K)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/RiskReturnSummaryUnlabeledBlackrockStrategicMunicipalOpportunitiesFundOfBlackrockMunicipalSeriesTrustClassK</Role>
      <ShortName>Risk/Return Summary- BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Class K)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="d51288d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>0022 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Class K)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/RiskReturnDetailDataElementsBlackrockStrategicMunicipalOpportunitiesFundOfBlackrockMunicipalSeriesTrustClassK</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Strategic Municipal Opportunities Fund of BlackRock Municipal Series Trust (Class K)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="d51288d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>040000 - Disclosure - Risk/Return Detail Data {Elements}</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/rr/role/RiskReturnDetailData</Role>
      <ShortName>Risk/Return Detail Data</ShortName>
      <MenuCategory></MenuCategory>
      <Position>8</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="485BPOS" original="d51288d485bpos.htm">d51288d485bpos.htm</File>
    <File>bmst-20200531.xsd</File>
    <File>bmst-20200531_cal.xml</File>
    <File>bmst-20200531_def.xml</File>
    <File>bmst-20200531_lab.xml</File>
    <File>bmst-20200531_pre.xml</File>
    <File>d51288dex99a14.htm</File>
    <File>d51288dex99a5.htm</File>
    <File>d51288dex99j1.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>g51288bar.jpg</File>
    <File>g51288c000011342_dgf.jpg</File>
    <File>g51288g1bar.jpg</File>
    <File>g51288g1c000011339_dgf.jpg</File>
    <File>g51288g1img0d01e9f21.gif</File>
    <File>g51288g2bar.jpg</File>
    <File>g51288g2c000011343_dgf.jpg</File>
    <File>g51288g2img13d8ba1d1.gif</File>
    <File>g51288img709b63e11.gif</File>
    <File>BarChart1.png</File>
    <File>BarChart2.png</File>
    <File>BarChart3.png</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/rr/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
