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Contingencies (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2014
Property and Casualty Group [Member]        
Loss Contingencies [Line Items]        
Increase in asbestos and environmental reserves $ 24us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= afg_PropertyAndCasualtyGroupMember
$ 54us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= afg_PropertyAndCasualtyGroupMember
$ 31us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= afg_PropertyAndCasualtyGroupMember
 
Liability for asbestos and environmental reserves       366us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= afg_PropertyAndCasualtyGroupMember
Reinsurance recoverables on asbestos and environmental reserves, net of allowance       77afg_ReinsuranceRecoverablesOnAsbestosAndEnvironmentalReserves
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= afg_PropertyAndCasualtyGroupMember
Former railroad and manufacturing operations [Member]        
Loss Contingencies [Line Items]        
Increase in environmental and other reserves 6us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= afg_FormerRailroadAndManufacturingOperationsMember
22us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= afg_FormerRailroadAndManufacturingOperationsMember
2us-gaap_AccrualForEnvironmentalLossContingenciesProvisionForNewLosses
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= afg_FormerRailroadAndManufacturingOperationsMember
 
American Premier and its subsidiaries [Member]        
Loss Contingencies [Line Items]        
Liability for environmental and other claims       80us-gaap_AccrualForEnvironmentalLossContingencies
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= afg_AmericanPremierAndItsSubsidiariesMember
GAFRI [Member]        
Loss Contingencies [Line Items]        
Liability for environmental and other claims       $ 10us-gaap_AccrualForEnvironmentalLossContingencies
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= afg_GafriMember