Long-Term Debt - Schedule of long-term debt (Details) - USD ($)	Sep. 30, 2020	May 29, 2020	Apr. 02, 2020	Dec. 31, 2019
Summary of Carrying value of long-term debt
Principal	$ 2,143,000,000			$ 1,493,000,000
Discount and Issue Costs	(35,000,000)			(20,000,000)
Carrying Value	2,108,000,000			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	(12,000,000)			(5,000,000)
Carrying Value	$ 1,306,000,000			1,013,000,000
Senior Notes | 4.50% Senior Notes due June 2047 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	4.50%
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
Senior Notes | 3.50% Senior Notes due August 2026 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	3.50%
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
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	$ 300,000,000		$ 300,000,000	0
Discount and Issue Costs	(7,000,000)			0
Carrying Value	293,000,000			0
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	825,000,000			475,000,000
Discount and Issue Costs	(23,000,000)			(15,000,000)
Carrying Value	$ 802,000,000			460,000,000
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	$ 200,000,000			0
Discount and Issue Costs	(4,000,000)			0
Carrying Value	$ 196,000,000			0
Subordinated Debentures | 5.125% Subordinate 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	$ 200,000,000			200,000,000
Discount and Issue Costs	(6,000,000)			(6,000,000)
Carrying Value	$ 194,000,000			194,000,000
Subordinated Debentures | 6% Subordinated Debentures due November 2055 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	6.00%
Summary of Carrying value of long-term debt
Principal	$ 150,000,000			150,000,000
Discount and Issue Costs	(5,000,000)			(5,000,000)
Carrying Value	$ 145,000,000			145,000,000
Subordinated Debentures | 5.625% Subordinated Debentures due June 2060 | AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.625%	5.625%
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
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	$ 125,000,000			125,000,000
Discount and Issue Costs	(4,000,000)			(4,000,000)
Carrying Value	$ 121,000,000			$ 121,000,000