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	Sep. 30, 2020	Dec. 31, 2019
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	$ 869	$ 1,026
Derivative liability, at fair value	3,665	3,735
MBS with embedded derivatives | Fixed maturities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	172	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	3,657	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	697	924
Equity index put options | Other liabilities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	0
Derivative liability, at fair value	4	1
Reinsurance contracts (embedded derivative) | Other liabilities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	$ 4	$ 4