Long-Term Debt - Schedule of long-term debt (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Summary of Carrying value of long-term debt
Principal	$ 1,993	$ 1,993
Discount and Issue Costs	(30)	(30)
Carrying Value	1,963	1,963
Senior Notes \| AFG
Summary of Carrying value of long-term debt
Principal	1,318	1,318
Discount and Issue Costs	(11)	(11)
Carrying Value	$ 1,307	1,307
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	590
Discount and Issue Costs	(2)	(2)
Carrying Value	$ 588	588
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	425
Discount and Issue Costs	(3)	(3)
Carrying Value	$ 422	422
Senior Notes \| 5.25% Senior Notes due April 2030 \| AFG
Debt Instrument [Line Items]
Interest rate on debt instruments	5.25%
Summary of Carrying value of long-term debt
Principal	$ 300	300
Discount and Issue Costs	(6)	(6)
Carrying Value	294	294
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% 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	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

Summary of Carrying value of long-term debt.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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