<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>20</ContextCount>
  <ElementCount>176</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>14</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report instance="d94963d485bpos.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/DocumentDocumentandEntityInformationElements</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory></MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>000021 - Document - Risk/Return Summary {Unlabeled} - BLACKROCK ADVANTAGE INTERNATIONAL FUND (Investor C Shares, Class R Shares, Institutional Shares and Investor A Shares)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DocumentRiskReturnSummaryUnlabeledBLACKROCKADVANTAGEINTERNATIONALFUNDInvestorCShares,ClassRShares,InstitutionalSharesandInvestorAShares</Role>
      <ShortName>Risk/Return Summary- BLACKROCK ADVANTAGE INTERNATIONAL FUND (Investor C Shares, Class R Shares, Institutional Shares and Investor A Shares)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>000022 - Schedule - Shareholder Fees</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleShareholderFees000022</Role>
      <ShortName>Shareholder Fees</ShortName>
      <MenuCategory></MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>000023 - Schedule - Annual Fund Operating Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleAnnualFundOperatingExpenses000023</Role>
      <ShortName>Annual Fund Operating Expenses</ShortName>
      <MenuCategory></MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>000024 - Schedule - Expense Example</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleExpenseExample000024</Role>
      <ShortName>Expense Example</ShortName>
      <MenuCategory></MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>000025 - Schedule - Expense Example, No Redemption</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleExpenseExampleNoRedemption000025</Role>
      <ShortName>Expense Example, No Redemption</ShortName>
      <MenuCategory></MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>000026 - Schedule - Annual Total Returns - BLACKROCK ADVANTAGE INTERNATIONAL FUND (Investor C Shares, Class R Shares, Institutional Shares and Investor A Shares) [BarChart]</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleAnnualTotalReturnsBarChart000026</Role>
      <ShortName>Annual Total Returns - BLACKROCK ADVANTAGE INTERNATIONAL FUND (Investor C Shares, Class R Shares, Institutional Shares and Investor A Shares) [BarChart]</ShortName>
      <MenuCategory></MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>000027 - Schedule - Average Annual Total Returns</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleAverageAnnualTotalReturns000027</Role>
      <ShortName>Average Annual Total Returns</ShortName>
      <MenuCategory></MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>000029 - Disclosure - Risk/Return Detail Data {Elements} - BLACKROCK ADVANTAGE INTERNATIONAL FUND (Investor C Shares, Class R Shares, Institutional Shares and Investor A Shares)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBLACKROCKADVANTAGEINTERNATIONALFUNDInvestorCShares,ClassRShares,InstitutionalSharesandInvestorAShares</Role>
      <ShortName>Risk/Return Detail Data- BLACKROCK ADVANTAGE INTERNATIONAL FUND (Investor C Shares, Class R Shares, Institutional Shares and Investor A Shares)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>000041 - Document - Risk/Return Summary {Unlabeled} - BLACKROCK ADVANTAGE INTERNATIONAL FUND (SERVICE CLASS and Investor A Shares For Participants)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DocumentRiskReturnSummaryUnlabeledBLACKROCKADVANTAGEINTERNATIONALFUNDSERVICECLASSandInvestorASharesForParticipants</Role>
      <ShortName>Risk/Return Summary- BLACKROCK ADVANTAGE INTERNATIONAL FUND (SERVICE CLASS and Investor A Shares For Participants)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>000046 - Schedule - Annual Total Returns - BLACKROCK ADVANTAGE INTERNATIONAL FUND (SERVICE CLASS and Investor A Shares For Participants) [BarChart]</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleAnnualTotalReturnsBarChart000046</Role>
      <ShortName>Annual Total Returns - BLACKROCK ADVANTAGE INTERNATIONAL FUND (SERVICE CLASS and Investor A Shares For Participants) [BarChart]</ShortName>
      <MenuCategory></MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>000047 - Schedule - Average Annual Total Returns</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleAverageAnnualTotalReturns000047</Role>
      <ShortName>Average Annual Total Returns</ShortName>
      <MenuCategory></MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>000049 - Disclosure - Risk/Return Detail Data {Elements} - BLACKROCK ADVANTAGE INTERNATIONAL FUND (SERVICE CLASS and Investor A Shares For Participants)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBLACKROCKADVANTAGEINTERNATIONALFUNDSERVICECLASSandInvestorASharesForParticipants</Role>
      <ShortName>Risk/Return Detail Data- BLACKROCK ADVANTAGE INTERNATIONAL FUND (SERVICE CLASS and Investor A Shares For Participants)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>000061 - Document - Risk/Return Summary {Unlabeled} - BLACKROCK ADVANTAGE INTERNATIONAL FUND (Class K)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DocumentRiskReturnSummaryUnlabeledBLACKROCKADVANTAGEINTERNATIONALFUNDClassK</Role>
      <ShortName>Risk/Return Summary- BLACKROCK ADVANTAGE INTERNATIONAL FUND (Class K)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>000063 - Schedule - Annual Fund Operating Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleAnnualFundOperatingExpenses000063</Role>
      <ShortName>Annual Fund Operating Expenses</ShortName>
      <MenuCategory></MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>000064 - Schedule - Expense Example</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleExpenseExample000064</Role>
      <ShortName>Expense Example</ShortName>
      <MenuCategory></MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>000066 - Schedule - Annual Total Returns - BLACKROCK ADVANTAGE INTERNATIONAL FUND (Class K) [BarChart]</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleAnnualTotalReturnsBarChart000066</Role>
      <ShortName>Annual Total Returns - BLACKROCK ADVANTAGE INTERNATIONAL FUND (Class K) [BarChart]</ShortName>
      <MenuCategory></MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>000067 - Schedule - Average Annual Total Returns</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/ScheduleAverageAnnualTotalReturns000067</Role>
      <ShortName>Average Annual Total Returns</ShortName>
      <MenuCategory></MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="d94963d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>000069 - Disclosure - Risk/Return Detail Data {Elements} - BLACKROCK ADVANTAGE INTERNATIONAL FUND (Class K)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBLACKROCKADVANTAGEINTERNATIONALFUNDClassK</Role>
      <ShortName>Risk/Return Detail Data- BLACKROCK ADVANTAGE INTERNATIONAL FUND (Class K)</ShortName>
      <MenuCategory></MenuCategory>
      <Position>19</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="d94963d485bpos.htm">d94963d485bpos.htm</File>
    <File>bf12-20200925.xsd</File>
    <File>bf12-20200925_cal.xml</File>
    <File>bf12-20200925_def.xml</File>
    <File>bf12-20200925_lab.xml</File>
    <File>bf12-20200925_pre.xml</File>
    <File>d94963dex9910a.htm</File>
  </InputFiles>
  <SupplementalFiles>
    <File>g94963bar.jpg</File>
    <File>g94963c000023030_dgf.jpg</File>
    <File>g94963g1bar.jpg</File>
    <File>g94963g1img99d956641.gif</File>
    <File>g94963g1new1c000023030_dgf.jpg</File>
    <File>g94963g2bar.jpg</File>
    <File>g94963g2c000198216.jpg</File>
    <File>g94963g2imgab5b25c21.gif</File>
    <File>g94963img0cdee2ae1.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>
