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, 2021	Dec. 31, 2020
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	$ 1,032	$ 999
Derivative liability, at fair value	3,964	3,943
MBS with embedded derivatives | Fixed maturities
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	67	37
MBS with embedded derivatives | Assets of discontinued annuity operations | Annuity subsidiaries
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	152	137
Fixed-indexed and variable-indexed annuities (embedded derivative) | Liabilities of discontinued annuity operations | Annuity subsidiaries
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	3,954	3,933
Call option | Assets of discontinued annuity operations | Annuity subsidiaries
Derivatives included in AFG' s Balance Sheet at fair value
Derivative asset, at fair value	813	825
Equity index put options | Liabilities of discontinued annuity operations | Annuity subsidiaries
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	6	5
Reinsurance contract (embedded derivative) | Liabilities of discontinued annuity operations | Annuity subsidiaries
Derivatives included in AFG' s Balance Sheet at fair value
Derivative liability, at fair value	$ 4	$ 5