Fair Value Measurements - Narrative (Details) $ in Millions	Sep. 30, 2024 USD ($) professional
Fair Value Disclosures [Abstract]
AFG's internal investment professionals | professional	20
Level 3 assets as a percentage of total assets measured at fair value	9.00%
Percentage of level 3 equity investments that do not qualify for equity accounting	30.00%
Level 3 assets that are equity securities that do not qualify for equity accounting	$ 438
Percentage of level 3 assets that were priced using non-binding broker quotes	7.00%
Level 3 assets that were priced using non-binding broker quotes	$ 112
Percentage of level 3 assets that were priced by pricing services	1.00%
Level 3 assets that were priced by pricing services.	$ 9
Percentage of internally developed level 3 assets compared to total level 3 assets	62.00%
Internally developed level 3 assets	$ 912
Percentage of level three assets that were priced using a discounted cash flow approach	70.00%
Level three assets that were priced using a discounted cash flow approach	$ 642
Percentage of level three assets, equity securities, priced using broker quotes and internal models	19.00%
Level three assets, equity securities, priced using broker quotes and internal models	$ 177