Fair Value Measurements - Narrative (Details)	3 Months Ended
	Sep. 30, 2023 USD ($) professional	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill impairment loss	$ 26,000,000
AFG's internal investment professionals | professional	20
Percentage of level three assets that were priced using a discounted cash flow approach	68.00%
Level three assets that were priced using a discounted cash flow approach	$ 481,000,000
Percentage of level three assets, equity securities, priced using broker quotes and internal models	18.00%
Level three assets, equity securities, priced using broker quotes and internal models	$ 124,000,000
Verikai, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration possible to be paid, maximum			$ 50,000,000
Contingent consideration — acquisitions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of potential contingent consideration	2,000,000	$ 25,000,000
Contingent consideration — acquisitions | Verikai, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of potential contingent consideration	0	22,000,000
Change in fair value of contingent consideration liability	$ 22,000,000
Level 3
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	7.00%
Percentage of level 3 assets that were priced using non-binding broker quotes	10.00%
Level 3 assets that were priced using non-binding broker quotes	$ 113,000,000
Percentage of level 3 assets that were priced by pricing services	1.00%
Level 3 assets that were priced by pricing services.	$ 13,000,000
Percentage of level 3 equity investments that do not qualify for equity accounting	28.00%
Level 3 assets that are equity securities that do not qualify for equity accounting	$ 322,000,000
Percentage of internally developed level 3 assets compared to total level 3 assets	61.00%
Internally developed level 3 assets	$ 706,000,000
Level 3 | Contingent consideration — acquisitions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of potential contingent consideration	$ 2,000,000	$ 25,000,000