Fair Value Measurements - Narrative (Details) $ in Millions	Sep. 30, 2021 USD ($) professional
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
AFG's internal investment professionals | professional	20
Level 3 | Continuing operations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 assets as a percentage of total assets measured at fair value	5.00%
Percentage of level 3 assets that were priced using non-binding broker quotes	17.00%
Level 3 assets that were priced using non-binding broker quotes	$ 150
Level 3 assets that were priced by pricing services.	$ 61
Percentage of level 3 assets that were priced by pricing services	7.00%
Percentage of level 3 equity investments that do not qualify for equity accounting	17.00%
Level 3 assets that are equity securities that do not qualify for equity accounting	$ 149
Internally developed level 3 assets	$ 519
Percentage of internally developed level 3 assets compared to total level 3 assets	58.00%