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Fair Value Measurements (Details Textual) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
professional
Mar. 31, 2014
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 75.00%afg_PercentageOfLevelThreeAssetsThatWerePricedUsingNonBindingBrokerQuotes  
Level 3 assets that were priced using non-binding broker quotes $ 990afg_LevelThreeAssetsThatWerePricedUsingNonBindingBrokerQuotes  
Percentage of internally developed Level 3 asset fair values to Shareholders' equity 6.00%afg_PercentageOfInternallyDevelopedLevelThreeAssetsToShareholdersEquity  
Total realized/unrealized gains (losses) included in net income on liabilities still outstanding   4afg_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
 
Fixed-indexed annuties (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 annuties (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
 
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 1,240us-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
 
Common stocks [Member]    
Fair Value Measurements (Textual) [Abstract]    
Number of stocks transferred from Level 2 to Level 1 1afg_NumberofStocksTransferredfromLevel2toLevel1DuringthePeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
 
Fair value of assets transferred from Level 2 to Level 1 53us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonStockMember
 
Perpetual preferred stocks [Member]    
Fair Value Measurements (Textual) [Abstract]    
Number of stocks transferred from Level 2 to Level 1 2afg_NumberofStocksTransferredfromLevel2toLevel1DuringthePeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_PreferredStockMember
 
Fair value of assets transferred from Level 2 to Level 1 5us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
/ us-gaap_StatementClassOfStockAxis
= us-gaap_PreferredStockMember
 
Number of stocks transferred from Level 1 to Level 2   8afg_NumberofStocksTransferredfromLevel1toLevel2DuringthePeriod
/ us-gaap_StatementClassOfStockAxis
= us-gaap_PreferredStockMember
Fair value of assets transferred from Level 1 to Level 2   55us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
/ us-gaap_StatementClassOfStockAxis
= us-gaap_PreferredStockMember
Run-off long-term care insurance business [Member]    
Fair Value Measurements (Textual) [Abstract]    
Estimated sale proceeds of the run-off long-term care insurance business $ 14afg_DisposalGroupEstimatedProceedsfromDivestitureofBusinessesNet
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= afg_DisposalGroupsIncludingDiscontinuedOperationsLongtermCareBusinessMember
[1]  
[1] Includes fair value of the potential additional consideration and is shown net of estimated expenses.