Derivatives (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	$ 18	$ 23
Not Designated as Hedging Instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	249	170
Derivative liability, at fair value	515	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	96	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	497	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	152	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	$ 18	$ 23