Derivatives (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	$ 26	$ 23
Not Designated as Hedging Instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	228	170
Derivative liability, at fair value	538	418
MBS with embedded derivatives [Member] | Fixed maturities [Member] | Not Designated as Hedging Instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	97	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	2	5
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	512	395
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	129	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	$ 26	$ 23