3.25.1 html 56 75 1 true 24 0 false 2 false false R1.htm 000000 - Document - Document and Entity Information {Elements} Sheet http://www.blackrock.com/role/DocumentDocumentandEntityInformationElements000000 Document and Entity Information Risk/Return 1 false false R2.htm 000021 - Disclosure - Risk/Return Detail Data {Elements} - iShares Municipal Bond Index Fund (Investor A, Institutional) Sheet http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsiSharesMunicipalBondIndexFundInvestorAInstitutional Risk/Return Detail Data- iShares Municipal Bond Index Fund (Investor A, Institutional) Risk/Return 2 false false R3.htm 000041 - Disclosure - Risk/Return Detail Data {Elements} - iShares Municipal Bond Index Fund (Class K) Sheet http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsiSharesMunicipalBondIndexFundClassK000041 Risk/Return Detail Data- iShares Municipal Bond Index Fund (Class K) Risk/Return 3 false false R4.htm 000061 - Disclosure - Risk/Return Detail Data {Elements} - iShares Municipal Bond Index Fund (Investor P) Sheet http://www.blackrock.com/role/DisclosureRiskReturnDetailDataElementsiSharesMunicipalBondIndexFundInvestorP Risk/Return Detail Data- iShares Municipal Bond Index Fund (Investor P) Risk/Return 4 false false All Reports Book All Reports bf11-20250428.xsd bf11-20250428_cal.xml bf11-20250428_def.xml bf11-20250428_lab.xml bf11-20250428_pre.xml d885005d485bpos.htm g885005bar.jpg g885005c000012345_dgf.jpg g885005g1bar.jpg g885005g1c000195949_dgf.jpg g885005g1img72b03c341.gif g885005g2bar.jpg g885005g2c789456123_dgf.jpg g885005g2img2db528a31.gif g885005imgea01076f1.gif http://xbrl.sec.gov/dei/2023 http://xbrl.sec.gov/rr/2023 true true