Derivatives (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	$ 16	$ 17
Not Designated as Hedging Instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	329	243
Derivative liability, at fair value	571	482
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	118	110
Public company warrants [Member] | Equity securities [Member] | Not Designated as Hedging Instrument [Member]
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	16
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	1
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	555	465
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	194	132
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	$ 16	$ 17