Long-Term Debt - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Nov. 15, 2020	Sep. 30, 2020	May 29, 2020	Apr. 02, 2020	Mar. 31, 2019
Long-term Debt, Fiscal Year Maturity [Abstract]
Scheduled principal payments on debt in next year	$ 0
Scheduled principal payments on debt in year two	0
Scheduled principal payments on debt in year three	0
Scheduled principal payments on debt in year four	0
Scheduled principal payments on debt in year five	0
Principal	1,993,000,000	$ 1,493,000,000
Cash interest payments on long-term debt	$ 83,000,000	65,000,000	$ 59,000,000
AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Revolving credit facility term	5 years
Revolving credit line	$ 500,000,000
Interest rate description for revolving credit facility	1.00% to 1.875% (currently 1.375%) over LIBOR
Amount borrowed under AFG revolving credit facility	$ 0	0
LIBOR | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on revolving debt facility	1.375%
LIBOR | AFG | Minimum
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on revolving debt facility	1.00%
LIBOR | AFG | Maximum
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on revolving debt facility	1.875%
Senior Notes | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 1,318,000,000	1,018,000,000
Senior Notes | 4.50% Senior Notes due June 2047 | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 590,000,000	590,000,000
Interest rate on debt instruments	4.50%
Senior Notes | 3.50% Senior Notes due August 2026 | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 425,000,000	425,000,000
Interest rate on debt instruments	3.50%
Senior Notes | 5.25% Senior Notes due April 2030 | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 300,000,000	0					$ 300,000,000
Interest rate on debt instruments	5.25%						5.25%
Subordinated Debt | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 675,000,000	475,000,000
Subordinated Debt | 5.125% Subordinated Debentures due December 2059 | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 200,000,000	200,000,000
Senior Notes, aggregate principal amount		$ 200,000,000
Interest rate on debt instruments	5.125%	5.125%
Subordinated Debt | 6% Subordinated Debentures due November 2055 | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 0	$ 150,000,000		$ 150,000,000
Interest rate on debt instruments	6.00%			6.00%
Subordinated Debt | 5.875% Subordinated Debentures due March 2059 | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 125,000,000	125,000,000
Senior Notes, aggregate principal amount								$ 125,000,000
Interest rate on debt instruments	5.875%							5.875%
Subordinated Debt | 4.50% Subordinated Debentures due September 2060 | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 200,000,000	0			$ 200,000,000
Interest rate on debt instruments	4.50%				4.50%
Subordinated Debt | 5.625% Subordinated Debentures due June 2060 | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Principal	$ 150,000,000	$ 0				$ 150,000,000
Interest rate on debt instruments	5.625%					5.625%
Subordinated Debt | 6-1/4% Subordinated Debentures due September 2054 | AFG
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on debt instruments		6.25%
Senior Notes, amount of the original debt instrument that was redeemed		$ 150,000,000