<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.23.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>118</ContextCount>
  <ElementCount>70</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>39</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report instance="d509710d485bpos.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>Risk/Return</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="d509710d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>000021 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Defensive Advantage U.S. Fund (Investor A, Institutional)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBlackRockDefensiveAdvantageUSFundInvestorAInstitutional</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Defensive Advantage U.S. Fund (Investor A, Institutional)</ShortName>
      <MenuCategory>Risk/Return</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="d509710d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>000041 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Defensive Advantage International Fund (Investor A, Institutional)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBlackRockDefensiveAdvantageInternationalFundInvestorAInstitutional</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Defensive Advantage International Fund (Investor A, Institutional)</ShortName>
      <MenuCategory>Risk/Return</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="d509710d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>000061 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Defensive Advantage Emerging Markets Fund (Investor A, Institutional)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBlackRockDefensiveAdvantageEmergingMarketsFundInvestorAInstitutional</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Defensive Advantage Emerging Markets Fund (Investor A, Institutional)</ShortName>
      <MenuCategory>Risk/Return</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="d509710d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>000081 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Defensive Advantage U.S. Fund (Class K)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBlackRockDefensiveAdvantageU.S.FundClassK000081</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Defensive Advantage U.S. Fund (Class K)</ShortName>
      <MenuCategory>Risk/Return</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="d509710d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>000101 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Defensive Advantage International Fund (Class K)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBlackRockDefensiveAdvantageInternationalFundClassK000101</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Defensive Advantage International Fund (Class K)</ShortName>
      <MenuCategory>Risk/Return</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="d509710d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>000121 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Defensive Advantage Emerging Markets Fund (Class K)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBlackRockDefensiveAdvantageEmergingMarketsFundClassK000121</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Defensive Advantage Emerging Markets Fund (Class K)</ShortName>
      <MenuCategory>Risk/Return</MenuCategory>
      <Position>7</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="d509710d485bpos.htm">d509710d485bpos.htm</File>
    <File>bf12-20230825.xsd</File>
    <File>bf12-20230825_cal.xml</File>
    <File>bf12-20230825_def.xml</File>
    <File>bf12-20230825_lab.xml</File>
    <File>bf12-20230825_pre.xml</File>
    <File>d509710dex9910a.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>g509710bar.jpg</File>
    <File>g509710c09260j732_dgf.jpg</File>
    <File>g509710c09260j765_dgf.jpg</File>
    <File>g509710c09260j799_dgf.jpg</File>
    <File>g509710g1bar.jpg</File>
    <File>g509710g1c09260j716_dgf.jpg</File>
    <File>g509710g1c09260j740_dgf.jpg</File>
    <File>g509710g1c09260j773_dgf.jpg</File>
    <File>g509710g1img583719871.gif</File>
    <File>g509710img509247301.gif</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="8">http://xbrl.sec.gov/dei/2023</BaseTaxonomy>
    <BaseTaxonomy items="517">http://xbrl.sec.gov/rr/2023</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
