Fair Value Measurements - Narrative (Details)	3 Months Ended	12 Months Ended
	Sep. 30, 2023 USD ($)	Dec. 31, 2024 USD ($) professional	Dec. 31, 2023 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
Level 3 assets as a percentage of total assets measured at fair value		7.00%
Internally developed Level 3 assets		$ 932,000,000
Percentage of internally developed Level 3 assets compared to total Level 3 assets		85.00%
Percentage of Level 3 assets that were priced using a discounted cash flow approach		69.00%
Level 3 assets that were priced using a discounted cash flow approach		$ 647,000,000
Level 3 assets that were priced using unobservable internal models and third party inputs		$ 285,000,000
Percentage of level 3 assets that were priced using financial information from third parties		11.00%
Level 3 assets that were priced using financial information from third parties		$ 118,000,000
Percentage of level 3 assets that were priced using non-binding broker quotes or pricing services		4.00%
Level 3 assets that were priced using non-binding broker quotes or pricing services		$ 46,000,000
Contingent consideration - acquisitions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration — acquisitions		$ 2,000,000	$ 2,000,000
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
Verikai, Inc. | Contingent consideration - acquisitions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration — acquisitions	0
Gain on change in fair value of contingent consideration liability	$ 22,000,000