<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.21.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>19</ContextCount>
  <ElementCount>176</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>13</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report instance="d153236d485bpos.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="d153236d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>000021 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Technology Opportunities Fund (Investor A, C, Institutional and Class R)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBlackRockTechnologyOpportunitiesFundInvestorACInstitutionalandClassR</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Technology Opportunities Fund (Investor A, C, Institutional and Class R)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="d153236d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>000041 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Technology Opportunities Fund (Service)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBlackRockTechnologyOpportunitiesFundService</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Technology Opportunities Fund (Service)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="d153236d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>000061 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock Technology Opportunities Fund (Class K)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBlackRockTechnologyOpportunitiesFundClassK000061</Role>
      <ShortName>Risk/Return Detail Data- BlackRock Technology Opportunities Fund (Class K)</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>
  <InputFiles>
    <File doctype="485BPOS" original="d153236d485bpos.htm">d153236d485bpos.htm</File>
    <File>bf12-20210923.xsd</File>
    <File>bf12-20210923_cal.xml</File>
    <File>bf12-20210923_def.xml</File>
    <File>bf12-20210923_lab.xml</File>
    <File>bf12-20210923_pre.xml</File>
    <File>d153236dex9910a.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>g153236bar.jpg</File>
    <File>g153236c091929570_dgf.jpg</File>
    <File>g153236g1bar.jpg</File>
    <File>g153236g1c091929620_dgf.jpg</File>
    <File>g153236g1imga0fab6b51.gif</File>
    <File>g153236g2bar.jpg</File>
    <File>g153236g2c000216612_dgf.jpg</File>
    <File>g153236g2imgebd804081.gif</File>
    <File>g153236img4f54431b1.gif</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>
