Long-Term Debt - Schedule of long-term debt (Details) - USD ($)	Dec. 31, 2020	Nov. 15, 2020	Sep. 30, 2020	May 29, 2020	Apr. 02, 2020	Dec. 31, 2019	Mar. 31, 2019
Summary of Carrying value of long-term debt
Principal	$ 1,993,000,000					$ 1,493,000,000
Discount and Issue Costs	(30,000,000)					(20,000,000)
Carrying Value	1,963,000,000					1,473,000,000
AFG
Summary of Carrying value of long-term debt
Carrying Value	1,963,000,000					1,473,000,000
Senior Notes | AFG
Summary of Carrying value of long-term debt
Principal	1,318,000,000					1,018,000,000
Discount and Issue Costs	(11,000,000)					(5,000,000)
Carrying Value	1,307,000,000					1,013,000,000
Senior Notes | 4.50% Senior Notes due June 2047 | AFG
Summary of Carrying value of long-term debt
Principal	590,000,000					590,000,000
Discount and Issue Costs	(2,000,000)					(2,000,000)
Carrying Value	$ 588,000,000					588,000,000
Interest rate on debt instruments	4.50%
Senior Notes | 3.50% Senior Notes due August 2026 | AFG
Summary of Carrying value of long-term debt
Principal	$ 425,000,000					425,000,000
Discount and Issue Costs	(3,000,000)					(3,000,000)
Carrying Value	$ 422,000,000					422,000,000
Interest rate on debt instruments	3.50%
Senior Notes | 5.25% Senior Notes due April 2030 | AFG
Summary of Carrying value of long-term debt
Principal	$ 300,000,000				$ 300,000,000	0
Discount and Issue Costs	(6,000,000)					0
Carrying Value	$ 294,000,000					0
Interest rate on debt instruments	5.25%				5.25%
Senior Notes | Other | AFG
Summary of Carrying value of long-term debt
Principal	$ 3,000,000					3,000,000
Discount and Issue Costs	0					0
Carrying Value	3,000,000					3,000,000
Subordinated Debentures | AFG
Summary of Carrying value of long-term debt
Principal	675,000,000					475,000,000
Discount and Issue Costs	(19,000,000)					(15,000,000)
Carrying Value	656,000,000					460,000,000
Subordinated Debentures | 4.50% Subordinated Debentures due September 2060 | AFG
Summary of Carrying value of long-term debt
Principal	200,000,000		$ 200,000,000			0
Discount and Issue Costs	(5,000,000)					0
Carrying Value	$ 195,000,000					0
Interest rate on debt instruments	4.50%		4.50%
Subordinated Debentures | 5.125% Subordinated Debentures due December 2059 | AFG
Summary of Carrying value of long-term debt
Principal	$ 200,000,000					200,000,000
Discount and Issue Costs	(6,000,000)					(6,000,000)
Carrying Value	$ 194,000,000					$ 194,000,000
Interest rate on debt instruments	5.125%					5.125%
Subordinated Debentures | 5.625% Subordinated Debentures due June 2060 | AFG
Summary of Carrying value of long-term debt
Principal	$ 150,000,000			$ 150,000,000		$ 0
Discount and Issue Costs	(4,000,000)					0
Carrying Value	$ 146,000,000					0
Interest rate on debt instruments	5.625%			5.625%
Subordinated Debentures | 5.875% Subordinated Debentures due March 2059 | AFG
Summary of Carrying value of long-term debt
Principal	$ 125,000,000					125,000,000
Discount and Issue Costs	(4,000,000)					(4,000,000)
Carrying Value	$ 121,000,000					121,000,000
Interest rate on debt instruments	5.875%						5.875%
Subordinated Debentures | 6% Subordinated Debentures due November 2055 | AFG
Summary of Carrying value of long-term debt
Principal	$ 0	$ 150,000,000				150,000,000
Discount and Issue Costs	0					(5,000,000)
Carrying Value	$ 0					$ 145,000,000
Interest rate on debt instruments	6.00%	6.00%