Long-Term Debt - Schedule of long-term debt (Details) - USD ($) $ in Millions	Dec. 31, 2016	Nov. 10, 2016	Dec. 31, 2015
Summary of Carrying value of long-term debt
Principal	$ 1,308		$ 1,020
Discount and Issue Costs	(25)		(22)
Carrying Value	1,283		998
AFG
Summary of Carrying value of long-term debt
Carrying Value	1,283		986
Senior Notes | AFG
Summary of Carrying value of long-term debt
Principal	1,008		708
Discount and Issue Costs	(15)		(12)
Carrying Value	993		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 | 3.50% Senior Notes due August 2026 | AFG
Summary of Carrying value of long-term debt
Principal	300		0
Discount and Issue Costs	(3)		0
Carrying Value	297		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	0	$ 18	12
Discount and Issue Costs	0		0
Carrying Value	$ 0		$ 12