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Long-Term Debt (Details Textual) - USD ($)
9 Months Ended
Sep. 30, 2015
Sep. 30, 2015
Dec. 31, 2014
Long-term Debt, Fiscal Year Maturity [Abstract]      
Scheduled principal payments on debt in 2015 (are less than) $ 1,000,000 $ 1,000,000  
Scheduled principal payments on debt in 2016 10,000,000 10,000,000  
Scheduled principal payments on debt in 2017 12,000,000 12,000,000  
Scheduled principal payments on debt in 2018 0 0  
Scheduled principal payments on debt in 2019 350,000,000 350,000,000  
Scheduled principal payments on debt in 2020 0 0  
Scheduled principal payments on debt after 2020 508,000,000 508,000,000  
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 1.28%)  
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 1.28% 1.28%  
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 8.4362% 8.4362%  
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 $ 7,000,000 $ 7,000,000  
Senior Notes [Member] | 9-7/8% Senior Notes due June 2019 [Member]      
Long-term Debt, Fiscal Year Maturity [Abstract]      
Stated interest rate, percentage 9.875% 9.875%  
Senior Notes [Member] | 9-7/8% Senior Notes due June 2019 [Member] | AFG [Member]      
Long-term Debt, Fiscal Year Maturity [Abstract]      
Senior Notes $ 350,000,000 $ 350,000,000 350,000,000
Stated interest rate, percentage 9.875% 9.875%  
Senior Notes [Member] | 6-3/8% Senior Notes due June 2042 [Member] | AFG [Member]      
Long-term Debt, Fiscal Year Maturity [Abstract]      
Senior Notes $ 230,000,000 $ 230,000,000 230,000,000
Stated interest rate, percentage 6.375% 6.375%  
Senior Notes [Member] | 5-3/4% Senior Notes due August 2042 [Member] | AFG [Member]      
Long-term Debt, Fiscal Year Maturity [Abstract]      
Senior Notes $ 125,000,000 $ 125,000,000 125,000,000
Stated interest rate, percentage 5.75% 5.75%  
Senior Notes [Member] | 7% Senior Notes due September 2050 [Member]      
Long-term Debt, Fiscal Year Maturity [Abstract]      
Senior Notes $ 132,000,000 $ 132,000,000  
Senior Notes [Member] | 7% Senior Notes due September 2050 [Member] | AFG [Member]      
Long-term Debt, Fiscal Year Maturity [Abstract]      
Senior Notes $ 0 $ 0 $ 132,000,000
Stated interest rate, percentage 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]      
Stated interest rate, percentage 6.25% 6.25%