Deferred Policy Acquisition Costs (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Deferred Policy Acquisition Costs Disclosures [Abstract]
Accumulated amortization of present value of future profits	$ 184	$ 195
Approximate rate of decrease in present value of future profits during next five years	0.167

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