Derivatives (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	$ 17	$ 23
Not designated as hedging instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	243	170
Derivative liability, at fair value	482	384
MBS with embedded derivatives [Member] | Fixed maturity [Member] | Not designated as hedging instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	110	99
Interest rate swaptions [Member] | Other investments [Member] | Not designated as hedging instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	1	5
Fixed-indexed annuities (embedded derivative) [Member] | Annuity benefits accumulated [Member] | Not designated as hedging instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	465	361
Equity index call options [Member] | Other investments [Member] | Not designated as hedging instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	132	66
Reinsurance contracts (embedded derivative) [Member] | Other liabilities [Member] | Not designated as hedging instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	$ 17	$ 23