<?xml version="1.0" encoding="utf-8"?>
<xbrl
  xmlns="http://www.xbrl.org/2003/instance"
  xmlns:afg="http://afginc.com/20210528"
  xmlns:dei="http://xbrl.sec.gov/dei/2021"
  xmlns:link="http://www.xbrl.org/2003/linkbase"
  xmlns:us-gaap="http://fasb.org/us-gaap/2021-01-31"
  xmlns:xbrldi="http://xbrl.org/2006/xbrldi"
  xmlns:xlink="http://www.w3.org/1999/xlink">
    <link:schemaRef xlink:href="afg-20210528.xsd" xlink:type="simple"/>
    <context id="c20210528to20210528">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001042046</identifier>
        </entity>
        <period>
            <startDate>2021-05-28</startDate>
            <endDate>2021-05-28</endDate>
        </period>
    </context>
    <context id="c20210528to20210528_StatementClassOfStockAxis_Four5SubordinatedDebenturesDueSeptember152060Member">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001042046</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">afg:Four5SubordinatedDebenturesDueSeptember152060Member</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2021-05-28</startDate>
            <endDate>2021-05-28</endDate>
        </period>
    </context>
    <context id="c20210528to20210528_StatementClassOfStockAxis_Five875SubordinatedDebenturesDueMarch302059Member">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001042046</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">afg:Five875SubordinatedDebenturesDueMarch302059Member</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2021-05-28</startDate>
            <endDate>2021-05-28</endDate>
        </period>
    </context>
    <context id="c20210528to20210528_StatementClassOfStockAxis_Five625SubordinatedDebenturesDueJune12060Member">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001042046</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">afg:Five625SubordinatedDebenturesDueJune12060Member</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2021-05-28</startDate>
            <endDate>2021-05-28</endDate>
        </period>
    </context>
    <context id="c20210528to20210528_StatementClassOfStockAxis_CommonStockMember">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001042046</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">us-gaap:CommonStockMember</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2021-05-28</startDate>
            <endDate>2021-05-28</endDate>
        </period>
    </context>
    <context id="c20210528to20210528_StatementClassOfStockAxis_Five125SubordinatedDebenturesDueDecember152059Member">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001042046</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">afg:Five125SubordinatedDebenturesDueDecember152059Member</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2021-05-28</startDate>
            <endDate>2021-05-28</endDate>
        </period>
    </context>
    <dei:AmendmentFlag
      contextRef="c20210528to20210528"
      id="Fact_2a40ae52acba4733b33c238082c19378">false</dei:AmendmentFlag>
    <dei:EntityFileNumber
      contextRef="c20210528to20210528"
      id="Fact_9186689de6e0440e95c7aa60cc9ace34">001-13653</dei:EntityFileNumber>
    <dei:EntityAddressStateOrProvince
      contextRef="c20210528to20210528"
      id="Fact_c9cd8e2401aa405abfbb21e6d72b8202">OH</dei:EntityAddressStateOrProvince>
    <dei:EntityCentralIndexKey
      contextRef="c20210528to20210528"
      id="Fact_084f2992358f49b3b3cff9c507a1aa36">0001042046</dei:EntityCentralIndexKey>
    <dei:DocumentType
      contextRef="c20210528to20210528"
      id="Fact_56d71aa384d448d18056e35e41174b59">8-K</dei:DocumentType>
    <dei:DocumentPeriodEndDate
      contextRef="c20210528to20210528"
      id="Fact_3124a061119048d6875683d0b823c9d2">2021-05-28</dei:DocumentPeriodEndDate>
    <dei:EntityRegistrantName
      contextRef="c20210528to20210528"
      id="Fact_2003bcf519e741b997cf6f653ba13b97">AMERICAN FINANCIAL GROUP, INC.</dei:EntityRegistrantName>
    <dei:EntityIncorporationStateCountryCode
      contextRef="c20210528to20210528"
      id="Fact_f1360bdf357d48598dd64f93c9f37c58">OH</dei:EntityIncorporationStateCountryCode>
    <dei:EntityTaxIdentificationNumber
      contextRef="c20210528to20210528"
      id="Fact_ff3f0c89b8c54c869ac820e8431c8ebd">31-1544320</dei:EntityTaxIdentificationNumber>
    <dei:EntityAddressAddressLine1
      contextRef="c20210528to20210528"
      id="Fact_bd957c0e05f84850a2fdc633dcaf66c2">301 East Fourth Street</dei:EntityAddressAddressLine1>
    <dei:EntityAddressCityOrTown
      contextRef="c20210528to20210528"
      id="Fact_251eacac4a224da4abd5bfd51bdb101d">Cincinnati</dei:EntityAddressCityOrTown>
    <dei:EntityAddressPostalZipCode
      contextRef="c20210528to20210528"
      id="Fact_88e48a7c1b83445b82f2b0c2d0642762">45202</dei:EntityAddressPostalZipCode>
    <dei:CityAreaCode
      contextRef="c20210528to20210528"
      id="Fact_2d1c2964cd0341e4828efbb0bcca90a9">513</dei:CityAreaCode>
    <dei:LocalPhoneNumber
      contextRef="c20210528to20210528"
      id="Fact_3ef1d3cff72945edafff0da27e4526ce">579-2121</dei:LocalPhoneNumber>
    <dei:WrittenCommunications
      contextRef="c20210528to20210528"
      id="Fact_d67c2c1263e1455fa22bcea4b49d7262">false</dei:WrittenCommunications>
    <dei:SolicitingMaterial
      contextRef="c20210528to20210528"
      id="Fact_d6a08057ab8b451697898a1f6b437a1c">false</dei:SolicitingMaterial>
    <dei:PreCommencementTenderOffer
      contextRef="c20210528to20210528"
      id="Fact_3e47bda72e7f4d7cb736ac11a8865bee">false</dei:PreCommencementTenderOffer>
    <dei:PreCommencementIssuerTenderOffer
      contextRef="c20210528to20210528"
      id="Fact_2dedeed0429149f59957c47864b41f66">false</dei:PreCommencementIssuerTenderOffer>
    <dei:EntityEmergingGrowthCompany
      contextRef="c20210528to20210528"
      id="Fact_d150d3525f9d495299a8a81a4da05cae">false</dei:EntityEmergingGrowthCompany>
    <dei:Security12bTitle
      contextRef="c20210528to20210528_StatementClassOfStockAxis_CommonStockMember"
      id="Fact_9111b3ed6fae4cceb965e6ffe6e60c80">Common Stock</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="c20210528to20210528_StatementClassOfStockAxis_CommonStockMember"
      id="Fact_9bdad31b64ed4f15a98c09c99e969e8a">AFG</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="c20210528to20210528_StatementClassOfStockAxis_CommonStockMember"
      id="Fact_50dc5839999f4f05910d3b9bc474f55f">NYSE</dei:SecurityExchangeName>
    <dei:Security12bTitle
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Five875SubordinatedDebenturesDueMarch302059Member"
      id="Fact_77c5e3e72fd6491fa2ece818cc70d8fa">5.875% Subordinated Debentures due March 30, 2059</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Five875SubordinatedDebenturesDueMarch302059Member"
      id="Fact_842f4137db8b4cc09448633f537a8b6d">AFGB</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Five875SubordinatedDebenturesDueMarch302059Member"
      id="Fact_89e2769da5a44a4a8024d2ebfd2e40c5">NYSE</dei:SecurityExchangeName>
    <dei:Security12bTitle
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Five125SubordinatedDebenturesDueDecember152059Member"
      id="Fact_137be29566994aa89cdf1ddda5b748ed">5.125% Subordinated Debentures due December 15, 2059</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Five125SubordinatedDebenturesDueDecember152059Member"
      id="Fact_ee4af1ac80df49e8b51fec5562d3de70">AFGC</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Five125SubordinatedDebenturesDueDecember152059Member"
      id="Fact_c416caac94d84c82a1f00d6ca2ea74e3">NYSE</dei:SecurityExchangeName>
    <dei:Security12bTitle
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Five625SubordinatedDebenturesDueJune12060Member"
      id="Fact_02e99db98987474cae98ef827ae531ad">5.625% Subordinated Debentures due June 1, 2060</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Five625SubordinatedDebenturesDueJune12060Member"
      id="Fact_8ca4d8cbfaf040b19212f91c46f8e896">AFGD</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Five625SubordinatedDebenturesDueJune12060Member"
      id="Fact_562509265eee47ba87e65b0285a04a96">NYSE</dei:SecurityExchangeName>
    <dei:Security12bTitle
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Four5SubordinatedDebenturesDueSeptember152060Member"
      id="Fact_6be15f9d50ac4ba0a29e2f89172dd581">4.5% Subordinated Debentures due September 15, 2060</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Four5SubordinatedDebenturesDueSeptember152060Member"
      id="Fact_20afb907ff834f65bb27eacde8a1d3cb">AFGE</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="c20210528to20210528_StatementClassOfStockAxis_Four5SubordinatedDebenturesDueSeptember152060Member"
      id="Fact_226b960c92e742a1ad21f32e728410c8">NYSE</dei:SecurityExchangeName>
</xbrl>
