<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.23.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>17</ContextCount>
  <ElementCount>76</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>11</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report instance="d453139d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000000 - Document - Document and Entity Information {Elements}</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DocumentDocumentandEntityInformationElements000000</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory></MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="d453139d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>000021 - Disclosure - Risk/Return Detail Data {Elements} - iShares Municipal Bond Index Fund (Investor A, Institutional)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsiSharesMunicipalBondIndexFundInvestorAInstitutional</Role>
      <ShortName>Risk/Return Detail Data- iShares Municipal Bond Index Fund (Investor A, Institutional)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="d453139d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>000041 - Disclosure - Risk/Return Detail Data {Elements} - iShares Municipal Bond Index Fund (Class K)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsiSharesMunicipalBondIndexFundClassK000041</Role>
      <ShortName>Risk/Return Detail Data- iShares Municipal Bond Index Fund (Class K)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="d453139d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>000061 - Disclosure - Risk/Return Detail Data {Elements} - iShares Municipal Bond Index Fund (Investor P)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsiSharesMunicipalBondIndexFundInvestorP</Role>
      <ShortName>Risk/Return Detail Data- iShares Municipal Bond Index Fund (Investor P)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>4</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Warning">[dq-0542-Deprecated-Concept] Concept RiskNarrativeTextBlock in rr/2021 used in 3 facts was deprecated in rr/2023 as of 2023-01-31 and should not be used. d453139d485bpos.htm 260, 1477, 2416</Log>
    <Log type="Warning">[dq-0542-Deprecated-Concept] Concept RiskLoseMoney in rr/2021 used in 3 facts was deprecated in rr/2023 as of 2023-01-31 and should not be used. d453139d485bpos.htm 260, 1477, 2416</Log>
  </Logs>
  <InputFiles>
    <File doctype="485BPOS" original="d453139d485bpos.htm">d453139d485bpos.htm</File>
    <File>bf11-20230425.xsd</File>
    <File>bf11-20230425_cal.xml</File>
    <File>bf11-20230425_def.xml</File>
    <File>bf11-20230425_lab.xml</File>
    <File>bf11-20230425_pre.xml</File>
    <File>d453139dex9910a.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>g453139bar.jpg</File>
    <File>g453139c000012345_dgf.jpg</File>
    <File>g453139g1bar.jpg</File>
    <File>g453139g1c000195949_dgf.jpg</File>
    <File>g453139g1imge5e5b30b1.gif</File>
    <File>g453139g2bar.jpg</File>
    <File>g453139g2c789456123_dgf.jpg</File>
    <File>g453139g2imgee23b81c1.gif</File>
    <File>g453139img31a253801.gif</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="8">http://xbrl.sec.gov/dei/2021</BaseTaxonomy>
    <BaseTaxonomy items="224">http://xbrl.sec.gov/rr/2021</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
