Deferred Policy Acquisition Costs - Narrative (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Deferred Policy Acquisition Costs Disclosures [Abstract]
Accumulated amortization of present value of future profits	$ 147	$ 141

The cumulative amount of amortization of present value of future profits that has been recognized in the Income Statement.

No definition available.

Name:	afg_AccumulatedAmortizationOfPresentValueOfFutureProfits
Namespace Prefix:	afg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant