Derivatives - Derivatives that do not qualify for hedge accounting under GAAP included in the Balance Sheet at fair value (Details) - Not Designated as Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	$ 312	$ 1,026
Derivative liability, at fair value	3,109	3,735
MBS with embedded derivatives | Fixed maturities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	103	102
Fixed-indexed and variable-indexed annuities (embedded derivative) | Annuity benefits accumulated
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	3,099	3,730
Equity index call options | Equity index call options
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	209	924
Equity index put options | Other liabilities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	8	1
Reinsurance contracts (embedded derivative) | Other liabilities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	$ 2	$ 4