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Fair Value Measurements (Details Textual) (USD $)
12 Months Ended
Dec. 31, 2014
professional
Dec. 31, 2013
Dec. 31, 2012
Fair Value Measurements (Textual) [Abstract]      
AFG's internal investment professionals 20afg_NumberOfInternalInvestmentProfessionals    
Level 3 assets as a percentage of total assets measured at fair value 3.50%afg_LevelThreeAssetAsPercentageOfAssetMeasuredAtFairValue    
Percentage of level 3 assets that were priced using non-binding broker quotes 79.00%afg_PercentageOfLevelThreeAssetsThatWerePricedUsingNonBindingBrokerQuotes    
Level 3 assets that were priced using non-binding broker quotes $ 1,070,000,000afg_LevelThreeAssetsThatWerePricedUsingNonBindingBrokerQuotes    
Total realized/unrealized gains (losses) included in net income on liabilities still outstanding 50,000,000afg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarningsOnLiabilitiesStillOutstanding 7,000,000afg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarningsOnLiabilitiesStillOutstanding (125,000,000)afg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarningsOnLiabilitiesStillOutstanding
Maximum [Member]      
Fair Value Measurements (Textual) [Abstract]      
Percentage of internally developed Level 3 asset fair values to total assets (are less than) 1.00%afg_PercentageOfInternallyDevelopedLevelThreeAssetsToTotalAssets
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Percentage of internally developed Level 3 asset fair values to Shareholders' equity (are less than) 5.00%afg_PercentageOfInternallyDevelopedLevelThreeAssetsToShareholdersEquity
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Fixed-indexed annuities (embedded derivative) [Member]      
Fair Value Measurements (Textual) [Abstract]      
Fair value of derivatives in annuity benefits accumulated measured using a discounted cash flow approach 1,160,000,000us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeInstrumentRiskAxis
= afg_FixedIndexedAnnuitiesEmbeddedDerivativeMember
   
Fixed-indexed annuities (embedded derivative) [Member] | Minimum [Member]      
Fair Value Measurements (Textual) [Abstract]      
Unobservable input surrenders used in Level 3 fair value determination 4.00%afg_FairValueInputsSurrenders
/ us-gaap_DerivativeInstrumentRiskAxis
= afg_FixedIndexedAnnuitiesEmbeddedDerivativeMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Fixed-indexed annuities (embedded derivative) [Member] | Maximum [Member]      
Fair Value Measurements (Textual) [Abstract]      
Unobservable input surrenders used in Level 3 fair value determination 16.00%afg_FairValueInputsSurrenders
/ us-gaap_DerivativeInstrumentRiskAxis
= afg_FixedIndexedAnnuitiesEmbeddedDerivativeMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Fixed-indexed annuities (embedded derivative), majority of future years [Member] | Minimum [Member]      
Fair Value Measurements (Textual) [Abstract]      
Unobservable input surrenders used in Level 3 fair value determination 5.00%afg_FairValueInputsSurrenders
/ us-gaap_DerivativeInstrumentRiskAxis
= afg_FixedIndexedAnnuitiesEmbeddedDerivativeMajorityofFutureYearsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Fixed-indexed annuities (embedded derivative), majority of future years [Member] | Maximum [Member]      
Fair Value Measurements (Textual) [Abstract]      
Unobservable input surrenders used in Level 3 fair value determination 11.00%afg_FairValueInputsSurrenders
/ us-gaap_DerivativeInstrumentRiskAxis
= afg_FixedIndexedAnnuitiesEmbeddedDerivativeMajorityofFutureYearsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Not Designated as Hedging Instrument [Member] | Annuity benefits [Member] | Fixed-indexed annuities (embedded derivative) [Member]      
Fair Value Measurements (Textual) [Abstract]      
Total realized/unrealized gains (losses) included in net income for the embedded derivatives related to the unlocking of actuarial assumptions $ 58,000,000afg_DerivativeGainLossOnDerivativeAnnuityUnlocking
/ us-gaap_DerivativeInstrumentRiskAxis
= afg_FixedIndexedAnnuitiesEmbeddedDerivativeMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= afg_AnnuitybenefitsMember
$ 2,000,000afg_DerivativeGainLossOnDerivativeAnnuityUnlocking
/ us-gaap_DerivativeInstrumentRiskAxis
= afg_FixedIndexedAnnuitiesEmbeddedDerivativeMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= afg_AnnuitybenefitsMember
$ 36,000,000afg_DerivativeGainLossOnDerivativeAnnuityUnlocking
/ us-gaap_DerivativeInstrumentRiskAxis
= afg_FixedIndexedAnnuitiesEmbeddedDerivativeMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= afg_AnnuitybenefitsMember