Long-Term Debt - Schedule of long-term debt (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Summary of Carrying value of long-term debt
Principal	$ 1,428	$ 1,308
Discount and Issue Costs	(23)	(25)
Carrying Value	1,405	1,283
Senior Notes | AFG
Summary of Carrying value of long-term debt
Principal	1,128	1,008
Discount and Issue Costs	(13)	(15)
Carrying Value	1,115	993
Senior Notes | 9-7/8% Senior Notes due June 2019 | AFG
Summary of Carrying value of long-term debt
Principal	350	350
Discount and Issue Costs	(1)	(1)
Carrying Value	349	349
Senior Notes | 4.50% Senior Notes due June 2047 | AFG
Summary of Carrying value of long-term debt
Principal	350	0
Discount and Issue Costs	(5)	0
Carrying Value	345	0
Senior Notes | 3.50% Senior Notes due August 2026 | AFG
Summary of Carrying value of long-term debt
Principal	300	300
Discount and Issue Costs	(3)	(3)
Carrying Value	297	297
Senior Notes | 6-3/8% Senior Notes due June 2042 | AFG
Summary of Carrying value of long-term debt
Principal	0	230
Discount and Issue Costs	0	(7)
Carrying Value	0	223
Senior Notes | 5-3/4% Senior Notes due August 2042 | AFG
Summary of Carrying value of long-term debt
Principal	125	125
Discount and Issue Costs	(4)	(4)
Carrying Value	121	121
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