<?xml version="1.0" encoding="utf-8"?>
<FilingSummary xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Version>2.4.0.6</Version>
  <ProcessingTime />
  <ReportFormat>Html</ReportFormat>
  <ContextCount>32</ContextCount>
  <ElementCount>92</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>23</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DocumentDocumentandEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>000011 - Document - Risk/Return Summary {Unlabeled} - BLACKROCK ALL-CAP ENERGY &amp; RESOURCES PORTFOLIO (Investor and Institutional)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DocumentRiskReturnSummaryUnlabeledBLACKROCKALLCAPENERGYRESOURCESPORTFOLIOInvestorandInstitutional</Role>
      <ShortName>Risk/Return Summary  - BLACKROCK ALL-CAP ENERGY &amp; RESOURCES PORTFOLIO (Investor and Institutional)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>000019 - Disclosure - Risk/Return Detail Data {Elements} - BLACKROCK ALL-CAP ENERGY &amp; RESOURCES PORTFOLIO (Investor and Institutional)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBLACKROCKALLCAPENERGYRESOURCESPORTFOLIOInvestorandInstitutional</Role>
      <ShortName>Risk/Return Detail Data  - BLACKROCK ALL-CAP ENERGY &amp; RESOURCES PORTFOLIO (Investor and Institutional)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>000021 - Document - Risk/Return Summary {Unlabeled} - BLACKROCK ENERGY &amp; RESOURCES PORTFOLIO</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DocumentRiskReturnSummaryUnlabeledBLACKROCKENERGYRESOURCESPORTFOLIO</Role>
      <ShortName>Risk/Return Summary  - BLACKROCK ENERGY &amp; RESOURCES PORTFOLIO</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>000029 - Disclosure - Risk/Return Detail Data {Elements} - BLACKROCK ENERGY &amp; RESOURCES PORTFOLIO</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBLACKROCKENERGYRESOURCESPORTFOLIO</Role>
      <ShortName>Risk/Return Detail Data  - BLACKROCK ENERGY &amp; RESOURCES PORTFOLIO</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>000031 - Document - Risk/Return Summary {Unlabeled} - BlackRock World Gold Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DocumentRiskReturnSummaryUnlabeledBlackRockWorldGoldFund</Role>
      <ShortName>Risk/Return Summary  - BlackRock World Gold Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>000039 - Disclosure - Risk/Return Detail Data {Elements} - BlackRock World Gold Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBlackRockWorldGoldFund</Role>
      <ShortName>Risk/Return Detail Data  - BlackRock World Gold Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>000041 - Document - Risk/Return Summary {Unlabeled} - BLACKROCK ALL-CAP ENERGY &amp; RESOURCES PORTFOLIO (Service Shares)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DocumentRiskReturnSummaryUnlabeledBLACKROCKALLCAPENERGYRESOURCESPORTFOLIOServiceShares</Role>
      <ShortName>Risk/Return Summary  - BLACKROCK ALL-CAP ENERGY &amp; RESOURCES PORTFOLIO (Service Shares)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>000049 - Disclosure - Risk/Return Detail Data {Elements} - BLACKROCK ALL-CAP ENERGY &amp; RESOURCES PORTFOLIO (Service Shares)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsBLACKROCKALLCAPENERGYRESOURCESPORTFOLIOServiceShares</Role>
      <ShortName>Risk/Return Detail Data  - BLACKROCK ALL-CAP ENERGY &amp; RESOURCES PORTFOLIO (Service Shares)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.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>
    </Report>
    <Report>
      <IsDefault>true</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs />
  <InputFiles>
    <File>brf21-20130128.xml</File>
    <File>brf21-20130128.xsd</File>
    <File>brf21-20130128_cal.xml</File>
    <File>brf21-20130128_def.xml</File>
    <File>brf21-20130128_lab.xml</File>
    <File>brf21-20130128_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>BarChart1.jpg</File>
    <File>BarChart2.jpg</File>
    <File>BarChart3.jpg</File>
    <File>BarChart4.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies />
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
