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Investments (Details 6) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments    
Realized before impairments $ 23us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments $ 20us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
Realized — impairments (4)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities (1)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
Fixed maturities [Member]    
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments    
Realized before impairments 3us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember		 13us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
Realized — impairments (5)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember		 (1)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
Change in unrealized 218afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember		 316afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
Equity securities [Member]    
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments    
Realized before impairments 21us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 6us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Realized — impairments (2)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Change in unrealized 6afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 13afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Mortgage Loans and Other Investments [Member]    
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments    
Realized before impairments 0us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MortgageLoansAndOtherInvestmentsMember		 1us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MortgageLoansAndOtherInvestmentsMember
Realized — impairments 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MortgageLoansAndOtherInvestmentsMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MortgageLoansAndOtherInvestmentsMember
Change in unrealized 0afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MortgageLoansAndOtherInvestmentsMember		 0afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MortgageLoansAndOtherInvestmentsMember
Other, Including DPAC and Reserves on Annuity and Long-term Care [Member]    
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments    
Realized before impairments (1)us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_OtherIncludingDPACandReservesonAnnuityandLongtermCareMember		 0us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_OtherIncludingDPACandReservesonAnnuityandLongtermCareMember
Realized — impairments 3us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_OtherIncludingDPACandReservesonAnnuityandLongtermCareMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_OtherIncludingDPACandReservesonAnnuityandLongtermCareMember
Change in unrealized (137)afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_OtherIncludingDPACandReservesonAnnuityandLongtermCareMember		 (136)afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_OtherIncludingDPACandReservesonAnnuityandLongtermCareMember
Total Pretax [Member]    
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments    
Realized before impairments 23us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_PretaxMember		 20us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_PretaxMember
Realized — impairments (4)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_PretaxMember		 (1)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_PretaxMember
Change in unrealized 87afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_PretaxMember		 193afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_PretaxMember
Tax Effects [Member]    
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments    
Realized before impairments (8)us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_TaxEffectsMember		 (7)us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_TaxEffectsMember
Realized — impairments 1us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_TaxEffectsMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_TaxEffectsMember
Change in unrealized (30)afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_TaxEffectsMember		 (68)afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_TaxEffectsMember
Noncontrolling Interests [Member]    
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments    
Realized before impairments 0us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NoncontrollingInterestMember		 0us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NoncontrollingInterestMember
Realized — impairments 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NoncontrollingInterestMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NoncontrollingInterestMember
Change in unrealized (1)afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NoncontrollingInterestMember		 (3)afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NoncontrollingInterestMember
Marketable Securities [Member]    
Realized gains (losses) and changes in unrealized appreciation (depreciation) related to fixed maturity and equity security investments    
Realized before impairments 15us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MarketableSecuritiesMember		 13us-gaap_MarketableSecuritiesRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MarketableSecuritiesMember
Realized — impairments (3)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MarketableSecuritiesMember		 (1)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MarketableSecuritiesMember
Change in unrealized $ 56afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MarketableSecuritiesMember		 $ 122afg_ChangeInUnrealizedAppreciationDepreciation
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_MarketableSecuritiesMember