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	Dec. 31, 2020	Dec. 31, 2019
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	$ 999	$ 1,026
Derivative liability, at fair value	3,943	3,735
MBS with embedded derivatives | Fixed maturities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	174	102
Public company warrants | Equity securities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	0	0
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	3,933	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	825	924
Equity index put options | Other liabilities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	5	1
Reinsurance contract (embedded derivative) | Other liabilities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	$ 5	$ 4