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Long-Term Debt (Details Textual) (USD $)
0 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2014
Dec. 31, 2013
Debt Instrument [Line Items]      
Scheduled principal payments on debt in 2014 $ 1,000,000 $ 1,000,000  
Scheduled principal payments on debt in 2015 14,000,000 14,000,000  
Scheduled principal payments on debt in 2016 45,000,000 45,000,000  
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 after 2019 640,000,000 640,000,000  
AFG [Member]
     
Debt Instrument [Line Items]      
Revolving credit line 500,000,000 500,000,000  
Interest rate on revolving debt facility 1.375%    
Amount borrowed under AFG revolving credit facility 0 0 0
Interest rate description for revolving credit facility   1.00% to 1.875% (currently 1.375%) over LIBOR  
AFG [Member] | Minimum [Member]
     
Debt Instrument [Line Items]      
Interest rate on revolving debt facility 1.00%    
AFG [Member] | Maximum [Member]
     
Debt Instrument [Line Items]      
Interest rate on revolving debt facility 1.875%    
National Interstate [Member]
     
Debt Instrument [Line Items]      
Revolving credit line 100,000,000 100,000,000  
Interest rate on revolving debt facility 1.20%    
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 1.20%)  
National Interstate [Member] | Maximum [Member]
     
Debt Instrument [Line Items]      
Interest rate on revolving debt facility 0.875%    
9-7/8% Senior Notes due June 2019 [Member]
     
Debt Instrument [Line Items]      
Stated interest rate, percentage 9.875% 9.875%  
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
6-3/8% Senior Notes due June 2042 [Member]
     
Debt Instrument [Line Items]      
Stated interest rate, percentage 6.375% 6.375%  
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
5-3/4% Senior Notes due August 2042 [Member]
     
Debt Instrument [Line Items]      
Stated interest rate, percentage 5.75% 5.75%  
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
7% Senior Notes due September 2050 [Member]
     
Debt Instrument [Line Items]      
Stated interest rate, percentage 7.00% 7.00%  
7% Senior Notes due September 2050 [Member] | AFG [Member]
     
Debt Instrument [Line Items]      
Senior Notes 132,000,000 132,000,000 132,000,000
6-1/4% Subordinated Debentures due September 2054 [Member]
     
Debt Instrument [Line Items]      
Stated interest rate, percentage 6.25% 6.25%  
6-1/4% Subordinated Debentures due September 2054 [Member] | AFG [Member]
     
Debt Instrument [Line Items]      
Subordinated Debentures $ 150,000,000 $ 150,000,000 $ 0