3.20.4 html 19 182 1 true 15 0 false 2 false false R1.htm 000001 - Document - Document and Entity Information Sheet http://www.BNY.com/20201223/role/DocumentAndEntityInformation Document and Entity Information 1 false true R2.htm 020000 - Document - Risk/Return Summary {Unlabeled} - BNY Mellon AMT-Free Municipal Bond Fund Sheet http://www.BNY.com/20201223/role/DocumentRiskReturnSummaryUnlabeledBNYMellonAMTFreeMunicipalBondFund Risk/Return Summary- BNY Mellon AMT-Free Municipal Bond Fund 2 false false R9.htm 020007 - Disclosure - Risk/Return Detail Data {Elements} - BNY Mellon AMT-Free Municipal Bond Fund Sheet http://www.BNY.com/20201223/role/DisclosureRiskReturnDetailDataElementsBNYMellonAMTFreeMunicipalBondFund Risk/Return Detail Data- BNY Mellon AMT-Free Municipal Bond Fund 3 false true R10.htm 020008 - Document - Risk/Return Summary {Unlabeled} - BNY Mellon High Yield Municipal Bond Fund Sheet http://www.BNY.com/20201223/role/DocumentRiskReturnSummaryUnlabeledBNYMellonHighYieldMunicipalBondFund Risk/Return Summary- BNY Mellon High Yield Municipal Bond Fund 4 false false R17.htm 020015 - Disclosure - Risk/Return Detail Data {Elements} - BNY Mellon High Yield Municipal Bond Fund Sheet http://www.BNY.com/20201223/role/DisclosureRiskReturnDetailDataElementsBNYMellonHighYieldMunicipalBondFund Risk/Return Detail Data- BNY Mellon High Yield Municipal Bond Fund 5 false false R18.htm 040000 - Disclosure - Risk/Return Detail Data {Elements} Sheet http://xbrl.sec.gov/rr/role/RiskReturnDetailData Risk/Return Detail Data 6 false false All Reports Book All Reports ck0000878092-20201223.htm ck0000878092-20201223.xsd ck0000878092-20201223_cal.xml ck0000878092-20201223_def.xml ck0000878092-20201223_lab.xml ck0000878092-20201223_pre.xml d1expenselmtagrmt-amtfmbf.htm d2expenselmtgrmt-hymbf.htm fundindependentdirectorscode.htm j1-consentbnymafmbfbnymhymbf.htm n-rule18f3.htm p1ia045personalsecurities.htm poa-group32020.htm img_0964e3fa7d974f2.jpg img_2e38aa39f6bd4f3.jpg img_412e1cd8eaf74f2.jpg img_e73721f99bab4f3.jpg BarChart1.png BarChart2.png http://xbrl.sec.gov/rr/2018-01-31 http://xbrl.sec.gov/dei/2019-01-31 true true