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Investments (Details Textual) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Investment [Line Items]      
Percentage (based on amount of unrealized loss) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade 70.00%afg_PercentageBasedOnUnrealizedLossOfAvailableForSaleFixedMaturitiesThatAreInUnrealizedLossPositionAndRatedInvestmentGrade    
Percentage (based on fair value) of available for sale fixed maturities that are in an unrealized loss position and rated investment grade 85.00%afg_PercentageBasedOnFairValueOfAvailableForSaleFixedMaturitiesThatAreInUnrealizedLossPositionAndRatedInvestmentGrade    
Other than temporary impairment charges $ 36us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities $ 12us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities $ 26us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
Average life of ABS 4 years 6 months    
Average life of MBS 4 years 6 months    
Percentage increase in shareholders' equity 10.00%afg_InvestmentsInIndividualIssuersAsPercentageOfShareholdersEquity 10.00%afg_InvestmentsInIndividualIssuersAsPercentageOfShareholdersEquity  
Residential Mortgage Backed Securities [Member]      
Investment [Line Items]      
Non-credit related portion of other-than-temporary impairment charges taken for securities still owned 220afg_NonCreditRelatedPortionOfOttiChargesTakenForSecuritiesStillOwned
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
229afg_NonCreditRelatedPortionOfOttiChargesTakenForSecuritiesStillOwned
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Fixed maturities, Gross Unrealized, Losses (15)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
(28)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Securities with non-credit other-than-temporary impairment charges [Member]      
Investment [Line Items]      
Gross Unrealized, Gains 151us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_SecuritiesWithNonCreditOtherThanTemporaryImpairmentChargesMember
150us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_SecuritiesWithNonCreditOtherThanTemporaryImpairmentChargesMember
 
Gross Unrealized, Losses (8)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_SecuritiesWithNonCreditOtherThanTemporaryImpairmentChargesMember
(13)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= afg_SecuritiesWithNonCreditOtherThanTemporaryImpairmentChargesMember
 
Fixed maturities [Member]      
Investment [Line Items]      
Fixed maturities, Gross Unrealized, Losses (79)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
(305)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
 
Number of available for sale securities in an unrealized loss position 650afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
   
Other than temporary impairment charges 15us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
5us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
9us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedMaturitiesMember
Common stocks [Member]      
Investment [Line Items]      
Number of available for sale securities in an unrealized loss position 34afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
   
Other than temporary impairment charges 26us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
   
Equity securities, Gross Unrealized, Losses (25)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
(16)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
 
Preferred stocks [Member]      
Investment [Line Items]      
Number of available for sale securities in an unrealized loss position 19afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
   
Equity securities, Gross Unrealized, Losses (5)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
(10)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
 
Maximum [Member] | Residential Mortgage Backed Securities [Member]      
Investment [Line Items]      
Other than temporary impairment charges 1us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Maximum [Member] | Preferred stocks [Member]      
Investment [Line Items]      
Other than temporary impairment charges $ 1us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Number of available for sale securities in an unrealized loss position for twelve months or longer [Member] | Common stocks [Member]      
Investment [Line Items]      
Number of available for sale securities in an unrealized loss position 0afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition
/ us-gaap_InvestmentTypeAxis
= afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPositionTwelveMonthsOrLongerMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
   
Number of available for sale securities in an unrealized loss position for twelve months or longer [Member] | Preferred stocks [Member]      
Investment [Line Items]      
Number of available for sale securities in an unrealized loss position 8afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPosition
/ us-gaap_InvestmentTypeAxis
= afg_NumberOfAvailableForSaleSecuritiesInUnrealizedLossPositionTwelveMonthsOrLongerMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember