Long-Term Debt - Schedule of long-term debt (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Summary of Carrying value of long-term debt
Principal	$ 1,498	$ 1,498
Discount and Issue Costs	(23)	(23)
Carrying Value	1,475	1,475
Senior Notes | AFG
Summary of Carrying value of long-term debt
Principal	823	823
Discount and Issue Costs	(5)	(5)
Carrying Value	$ 818	$ 818
Senior Notes | 4.50% Senior Notes due June 2047 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	4.50%	4.50%
Summary of Carrying value of long-term debt
Principal	$ 567	$ 567
Discount and Issue Costs	(1)	(1)
Carrying Value	$ 566	$ 566
Senior Notes | 5.25% Senior Notes due April 2030 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.25%	5.25%
Summary of Carrying value of long-term debt
Principal	$ 253	$ 253
Discount and Issue Costs	(4)	(4)
Carrying Value	249	249
Senior Notes | Other | AFG
Summary of Carrying value of long-term debt
Principal	3	3
Discount and Issue Costs	0	0
Carrying Value	3	3
Subordinated Debentures | AFG
Summary of Carrying value of long-term debt
Principal	675	675
Discount and Issue Costs	(18)	(18)
Carrying Value	$ 657	657
Subordinated Debentures | 4.50% Subordinated Debentures due September 2060 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	4.50%
Summary of Carrying value of long-term debt
Principal	$ 200	200
Discount and Issue Costs	(5)	(5)
Carrying Value	$ 195	195
Subordinated Debentures | 5.125% Subordinated Debentures due December 2059 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.125%
Summary of Carrying value of long-term debt
Principal	$ 200	200
Discount and Issue Costs	(5)	(5)
Carrying Value	$ 195	195
Subordinated Debentures | 5.625% Subordinated Debentures due June 2060 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.625%
Summary of Carrying value of long-term debt
Principal	$ 150	150
Discount and Issue Costs	(4)	(4)
Carrying Value	$ 146	146
Subordinated Debentures | 5.875% Subordinated Debentures due March 2059 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.875%
Summary of Carrying value of long-term debt
Principal	$ 125	125
Discount and Issue Costs	(4)	(4)
Carrying Value	$ 121	$ 121