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Segments of Operations (Details Textual) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 3 Months Ended
Dec. 31, 2014
segment
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2012
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2012
Jun. 30, 2013
Segments of Operations (Textual) [Abstract]                
Number of segments 5us-gaap_NumberOfReportableSegments              
Revenue derived from sales of property and casualty insurance outside of the United States 5.00%afg_DisclosureOnGeographicAreasPropertyAndCasualtyRevenuesAttributedToForeignCountries 5.00%afg_DisclosureOnGeographicAreasPropertyAndCasualtyRevenuesAttributedToForeignCountries 5.00%afg_DisclosureOnGeographicAreasPropertyAndCasualtyRevenuesAttributedToForeignCountries          
Earnings (losses) of managed investment entities attributable to noncontrolling interests $ (51)afg_EarningsLossesOfVariableInterestEntitiesAttributableToNoncontrollingInterests [1] $ (26)afg_EarningsLossesOfVariableInterestEntitiesAttributableToNoncontrollingInterests [1] $ (98)afg_EarningsLossesOfVariableInterestEntitiesAttributableToNoncontrollingInterests [1]          
Run-off Long-term Care and Life [Member]                
Segments of Operations (Textual) [Abstract]                
Loss recognition charge on closed block of long-term care insurance 0afg_LossRecognitionCharge
/ us-gaap_StatementBusinessSegmentsAxis
= afg_RunOffLongTermCareAndLifeMember		 0afg_LossRecognitionCharge
/ us-gaap_StatementBusinessSegmentsAxis
= afg_RunOffLongTermCareAndLifeMember		   153afg_LossRecognitionCharge
/ us-gaap_StatementBusinessSegmentsAxis
= afg_RunOffLongTermCareAndLifeMember		        
Other [Member]                
Segments of Operations (Textual) [Abstract]                
Special charges to increase asbestos and environmental reserves         6afg_SpecialChargesToIncreaseAsbestosAndEnvironmentalReserves
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 22afg_SpecialChargesToIncreaseAsbestosAndEnvironmentalReserves
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 2afg_SpecialChargesToIncreaseAsbestosAndEnvironmentalReserves
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		  
Earnings (losses) of managed investment entities attributable to noncontrolling interests (51)afg_EarningsLossesOfVariableInterestEntitiesAttributableToNoncontrollingInterests
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 (26)afg_EarningsLossesOfVariableInterestEntitiesAttributableToNoncontrollingInterests
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 (98)afg_EarningsLossesOfVariableInterestEntitiesAttributableToNoncontrollingInterests
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		          
Loss on retirement of debt     (8)us-gaap_GainsLossesOnExtinguishmentOfDebt
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		          
Charge for a labor matter related to former operations     15afg_ChargeForLaborMatterRelatedToFormerOperations
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		          
Other Lines [Member]                
Segments of Operations (Textual) [Abstract]                
Special charges to increase asbestos and environmental reserves         24afg_SpecialChargesToIncreaseAsbestosAndEnvironmentalReserves
/ us-gaap_SubsegmentsAxis
= afg_OtherLinesMember		 54afg_SpecialChargesToIncreaseAsbestosAndEnvironmentalReserves
/ us-gaap_SubsegmentsAxis
= afg_OtherLinesMember		 31afg_SpecialChargesToIncreaseAsbestosAndEnvironmentalReserves
/ us-gaap_SubsegmentsAxis
= afg_OtherLinesMember		  
Insurance-related Assessments [Member] | Annuity [Member]                
Segments of Operations (Textual) [Abstract]                
Charge for guaranty fund assessments for the insolvency and liquidation of an unaffiliated life insurance company               $ 5us-gaap_LossContingencyLossInPeriod
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_InsuranceRelatedAssessmentsMember
/ us-gaap_StatementBusinessSegmentsAxis
= afg_AnnuityMember
[1] Included in Earnings before income taxes in AFG’s Statement of Earnings.