Long-Term Debt - Schedule of long-term debt (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Summary of Carrying value of long-term debt
Principal	$ 1,318	$ 1,318
Discount and Issue Costs	(17)	(17)
Carrying Value	1,301	1,301
Senior Notes | AFG
Summary of Carrying value of long-term debt
Principal	1,018	1,018
Discount and Issue Costs	(7)	(7)
Carrying Value	1,011	1,011
Senior Notes | 4.50% Senior Notes due June 2047 | AFG
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
Summary of Carrying value of long-term debt
Principal	425	425
Discount and Issue Costs	(5)	(5)
Carrying Value	420	420
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	300	300
Discount and Issue Costs	(10)	(10)
Carrying Value	290	290
Subordinated Debentures | 6-1/4% Subordinated Debentures due September 2054 | AFG
Summary of Carrying value of long-term debt
Principal	150	150
Discount and Issue Costs	(5)	(5)
Carrying Value	145	145
Subordinated Debentures | 6% Subordinated Debentures due November 2055 | AFG
Summary of Carrying value of long-term debt
Principal	150	150
Discount and Issue Costs	(5)	(5)
Carrying Value	$ 145	$ 145