Long-Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Summary of Carrying value of long-term debt
Principal	$ 1,020	$ 1,020
Debt Issue Costs	(22)	(22)
Carrying Value	998	998
Senior Notes [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Principal	708	708
Debt Issue Costs	(12)	(12)
Carrying Value	696	696
Senior Notes [Member] | 9-7/8% Senior Notes due June 2019 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Principal	350	350
Debt Issue Costs	(1)	(1)
Carrying Value	349	349
Senior Notes [Member] | 6-3/8% Senior Notes due June 2042 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Principal	230	230
Debt Issue Costs	(7)	(7)
Carrying Value	223	223
Senior Notes [Member] | 5-3/4% Senior Notes due August 2042 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Principal	125	125
Debt Issue Costs	(4)	(4)
Carrying Value	121	121
Senior Notes [Member] | Other [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Principal	3	3
Debt Issue Costs	0	0
Carrying Value	3	3
Subordinated Debentures [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Principal	300	300
Debt Issue Costs	(10)	(10)
Carrying Value	290	290
Subordinated Debentures [Member] | 6-1/4% Subordinated Debentures due September 2054 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Principal	150	150
Debt Issue Costs	(5)	(5)
Carrying Value	145	145
Subordinated Debentures [Member] | 6% Subordinated Debentures due November 2055 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Principal	150	150
Debt Issue Costs	(5)	(5)
Carrying Value	145	145
Revolving Credit Facility [Member] | National Interstate [Member]
Summary of Carrying value of long-term debt
Principal	12	12
Debt Issue Costs	0	0
Carrying Value	$ 12	$ 12