Long-Term Debt - Schedule of long-term debt (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Summary of Carrying value of long-term debt
Principal	$ 1,326	$ 1,020
Discount and Issue Costs	(26)	(22)
Carrying Value	1,300	998
Senior Notes | AFG
Summary of Carrying value of long-term debt
Principal	1,008	708
Discount and Issue Costs	(16)	(12)
Carrying Value	992	696
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 | 3-1/2% Senior Notes due August 2026 | AFG
Summary of Carrying value of long-term debt
Principal	300	0
Discount and Issue Costs	(4)	0
Carrying Value	296	0
Senior Notes | 6-3/8% Senior Notes due June 2042 | AFG
Summary of Carrying value of long-term debt
Principal	230	230
Discount and Issue Costs	(7)	(7)
Carrying Value	223	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
Revolving Credit Facility | National Interstate
Summary of Carrying value of long-term debt
Principal	18	12
Discount and Issue Costs	0	0
Carrying Value	$ 18	$ 12