Long-Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Summary of Carrying value of long-term debt
Long-term debt	$ 880	$ 1,061
Notes payable secured by real estate due 2015 through 2016	10	59
AFG [Member]
Summary of Carrying value of long-term debt
Other long-term debt	3	3
Long-term debt	708	840
Subsidiaries [Member]
Summary of Carrying value of long-term debt
Long-term debt	22	71
Notes payable secured by real estate due 2015 through 2016	10	59
National Interstate bank credit facility	12	12
Senior Notes [Member] | 9-7/8% Senior Notes due June 2019 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	350	350
Senior Notes [Member] | 6-3/8% Senior Notes due June 2042 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	230	230
Senior Notes [Member] | 5-3/4% Senior Notes due August 2042 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	125	125
Senior Notes [Member] | 7% Senior Notes due September 2050 [Member]
Summary of Carrying value of long-term debt
Senior Notes	132
Senior Notes [Member] | 7% Senior Notes due September 2050 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	0	132
Subordinated Debt [Member] | 6-1/4% Subordinated Debentures due September 2054 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Subordinated Debentures	$ 150	$ 150