Long-Term Debt (Details Textual) - USD ($)		12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Nov. 30, 2015	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014
Long-term Debt, Fiscal Year Maturity [Abstract]
Scheduled principal payments on debt in next year	$ 0	$ 0
Scheduled principal payments on debt in year two	12,000,000	12,000,000
Scheduled principal payments on debt in year three	0	0
Scheduled principal payments on debt in year four	350,000,000	350,000,000
Scheduled principal payments on debt in year five	0	0
Scheduled principal payments on debt after year five	658,000,000	658,000,000
Cash interest payments on long-term debt		75,000,000	$ 72,000,000	$ 71,000,000
AFG [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Revolving credit line	500,000,000	500,000,000
Amount borrowed under AFG revolving credit facility	0	$ 0	0
Interest rate description of fair value hedge activity		three-month LIBOR plus 8.099%
Interest rate description for revolving credit facility		1.00% to 1.875% (currently 1.375%) over LIBOR
National Interstate [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Revolving credit line	100,000,000	$ 100,000,000
Loans payable under National Interstate bank credit facility	$ 12,000,000	$ 12,000,000
Interest rate description for revolving credit facility		six-month LIBOR plus 0.875% (currently 1.399%)
LIBOR [Member] | AFG [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on revolving debt facility	1.375%
LIBOR [Member] | AFG [Member] | Minimum [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on revolving debt facility	1.00%
LIBOR [Member] | AFG [Member] | Maximum [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on revolving debt facility	1.875%
LIBOR [Member] | National Interstate [Member]
Debt Instrument [Line Items]
Effective interest rate, percentage	1.399%	1.399%
LIBOR [Member] | National Interstate [Member] | Maximum [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on revolving debt facility	0.875%
Interest rate swaps [Member] | Fair Value Hedge [Member] | Designated as Hedging Instrument [Member] | AFG [Member]
Debt Instrument [Line Items]
Effective interest rate, percentage	8.611%	8.611%
Long-term Debt, Fiscal Year Maturity [Abstract]
Cash interest received on long-term debt affiliated with interest rate swap		$ 2,000,000
Interest rate swaps [Member] | Fair Value Hedge [Member] | Designated as Hedging Instrument [Member] | LIBOR [Member] | AFG [Member]
Debt Instrument [Line Items]
Basis spread on variable interest rate	8.099%	8.099%
Long-term debt [Member] | Interest rate swaps [Member] | Fair Value Hedge [Member] | Designated as Hedging Instrument [Member] | AFG [Member]
Debt Instrument [Line Items]
Net derivatives, at fair value	$ 2,000,000	$ 2,000,000
Senior Notes [Member] | 9-7/8% Senior Notes due June 2019 [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on debt instruments	9.875%	9.875%					9.875%
Senior Notes [Member] | 9-7/8% Senior Notes due June 2019 [Member] | AFG [Member]
Debt Instrument [Line Items]
Senior Notes	$ 350,000,000	$ 350,000,000	350,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on debt instruments	9.875%	9.875%
Senior Notes [Member] | 6-3/8% Senior Notes due June 2042 [Member] | AFG [Member]
Debt Instrument [Line Items]
Senior Notes	$ 230,000,000	$ 230,000,000	230,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on debt instruments	6.375%	6.375%
Senior Notes [Member] | 5-3/4% Senior Notes due August 2042 [Member] | AFG [Member]
Debt Instrument [Line Items]
Senior Notes	$ 125,000,000	$ 125,000,000	125,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on debt instruments	5.75%	5.75%
Senior Notes [Member] | 7% Senior Notes due September 2050 [Member]
Debt Instrument [Line Items]
Senior Notes						$ 132,000,000
Senior Notes [Member] | 7% Senior Notes due September 2050 [Member] | AFG [Member]
Debt Instrument [Line Items]
Senior Notes	$ 0	$ 0	132,000,000
Long-term Debt, Fiscal Year Maturity [Abstract]
Interest rate on debt instruments	7.00%	7.00%				7.00%
Subordinated Debt [Member] | 6-1/4% Subordinated Debentures due September 2054 [Member] | AFG [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Subordinated Debentures	$ 150,000,000	$ 150,000,000	150,000,000					$ 150,000,000
Interest rate on debt instruments	6.25%	6.25%						6.25%
Subordinated Debt [Member] | 6% Subordinated Debentures due November 2055 [Member] | AFG [Member]
Long-term Debt, Fiscal Year Maturity [Abstract]
Subordinated Debentures	$ 150,000,000	$ 150,000,000	$ 0		$ 150,000,000
Interest rate on debt instruments	6.00%	6.00%			6.00%