Fair Value Measurements - Narrative (Details)	3 Months Ended	12 Months Ended
	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($) professional	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 06, 2021 USD ($)
Fair Value Measurements (Textual) [Abstract]
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		8.00%
Percentage of level 3 assets that were priced using non-binding broker quotes		9.00%
Level 3 assets that were priced using non-binding broker quotes		$ 111,000,000
Percentage of Level 3 assets that were priced by pricing services		1.00%
Level 3 assets that were priced by pricing services.		$ 9,000,000
Percentage of equity investments in Level 3 assets that do not qualify for equity accounting		28.00%
Equity investments in Level 3 assets that do not qualify for equity accounting		$ 345,000,000
Internally developed Level 3 assets		$ 765,000,000
Percentage of internally developed Level 3 assets compared to total Level 3 assets		62.00%
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		$ 528,000,000
Percentage of equity securities in Level 3 priced using broker quotes and internal models		18.00%
Equity securities in Level 3 priced using broker quotes and internal models		$ 139,000,000
Contingent consideration - acquisitions
Fair Value Measurements (Textual) [Abstract]
Contingent consideration — acquisitions		2,000,000	$ 25,000,000
Verikai, Inc.
Fair Value Measurements (Textual) [Abstract]
Contingent consideration possible to be paid, maximum				$ 50,000,000
Contingent consideration — acquisitions					$ 23,000,000
Verikai, Inc. | Contingent consideration - acquisitions
Fair Value Measurements (Textual) [Abstract]
Contingent consideration — acquisitions	0
Gain on change in fair value of contingent consideration liability	$ 22,000,000
Level 3 | Contingent consideration - acquisitions
Fair Value Measurements (Textual) [Abstract]
Contingent consideration — acquisitions		$ 2,000,000	$ 25,000,000