Subsequent Event (Details) (USD $) In Billions, unless otherwise specified	1 Months Ended
Oct. 31, 2012
Subsequent Events [Abstract]
Estimated total industry insured losses from catastrophic event	$ 20
Estimated losses from catastrophic event as a percentage of shareholders' equity	1.00%

Subsequent event, estimated losses from catastrophic event as percentage of Shareholders' Equity.

No definition available.

Name:	afg_SubsequentEventEstimatedLossesCatastrophicEventAsPercentageOfShareholdersEquity
Namespace Prefix:	afg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration