Long-Term Debt - Schedule of long-term debt (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Summary of Carrying value of long-term debt
Principal	$ 1,498		$ 1,521
Discount and Issue Costs	(24)		(25)
Carrying Value	1,474		1,496
Senior Notes | AFG
Summary of Carrying value of long-term debt
Principal	823		846
Discount and Issue Costs	(5)		(6)
Carrying Value	$ 818		840
Senior Notes | 4.50% Senior Notes due June 2047 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	4.50%	4.50%		4.50%
Summary of Carrying value of long-term debt
Principal	$ 567		582
Discount and Issue Costs	(1)		(1)
Carrying Value	$ 566		581
Senior Notes | 5.25% Senior Notes due April 2030 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.25%	5.25%		5.25%
Summary of Carrying value of long-term debt
Principal	$ 253		261
Discount and Issue Costs	(4)		(5)
Carrying Value	249		256
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	(19)		(19)
Carrying Value	$ 656		656
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	(6)		(6)
Carrying Value	$ 194		194
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