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Long-Term Debt (Details Textual) (USD $)
3 Months Ended 3 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2013
AFG [Member]
Dec. 31, 2012
AFG [Member]
Mar. 31, 2013
AFG [Member]
Minimum [Member]
Mar. 31, 2013
AFG [Member]
Maximum [Member]
Mar. 31, 2013
National Interstate [Member]
Mar. 31, 2013
National Interstate [Member]
Maximum [Member]
Nov. 30, 2012
National Interstate [Member]
Line of Credit [Member]
Mar. 31, 2013
9-7/8% Senior Notes due June 2019 [Member]
Mar. 31, 2013
6-3/8% Senior Notes due June 2042 [Member]
Mar. 31, 2013
5-3/4% Senior Notes due August 2042 [Member]
Mar. 31, 2013
7% Senior Notes due September 2050 [Member]
Debt Instrument [Line Items]                          
Scheduled principal payments on debt in 2013 $ 17,000,000                        
Scheduled principal payments on debt in 2014 2,000,000                        
Scheduled principal payments on debt in 2015 14,000,000                        
Scheduled principal payments on debt in 2016 45,000,000                        
Scheduled principal payments on debt in 2017 12,000,000                        
Scheduled principal payments on debt in 2018 0                        
Revolving credit line       500,000,000         100,000,000        
Interest rate on revolving debt facility     1.375%   1.00% 1.875% 1.34% 0.875%          
Amount borrowed under AFG revolving credit facility     0                    
Loans payable under National Interstate bank credit facility             12,000,000            
Interest rate description for revolving credit facility     1.00% to 1.875% (currently 1.375%) over LIBOR       LIBOR plus 0.875% (currently 1.340%)            
Stated interest rate, percentage                   9.875% 6.375% 5.75% 7.00%
Secured Borrowings guaranteed by AFG $ 15,000,000 $ 16,000,000