Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Summary of Carrying value of long-term debt
Long-term debt	$ 1,062	$ 913
AFG [Member]
Summary of Carrying value of long-term debt
Other long-term debt	3	3
Long-term debt	840	840
Subsidiaries [Member]
Summary of Carrying value of long-term debt
Long-term debt	72	73
Notes payable secured by real estate due 2014 through 2016	60	61
National Interstate bank credit facility	12	12
9-7/8% Senior Notes due June 2019 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	350	350
6-3/8% Senior Notes due June 2042 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	230	230
5-3/4% Senior Notes due August 2042 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	125	125
7% Senior Notes due September 2050 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Senior Notes	132	132
6-1/4% Subordinated Debentures due September 2054 [Member] | AFG [Member]
Summary of Carrying value of long-term debt
Subordinated Debentures	$ 150	$ 0