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Long-Term Debt (Details Textual) - USD ($)
1 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2016
Jun. 30, 2016
Dec. 31, 2015
Long-term Debt, Fiscal Year Maturity [Abstract]        
Scheduled principal payments on debt in 2016 $ 0 $ 0 $ 0  
Scheduled principal payments on debt in 2017 12,000,000 12,000,000 12,000,000  
Scheduled principal payments on debt in 2018 0 0 0  
Scheduled principal payments on debt in 2019 350,000,000 350,000,000 350,000,000  
Scheduled principal payments on debt in 2020 0 0 0  
Scheduled principal payments on debt in 2021 0 0 0  
Scheduled principal payments on debt after 2021 658,000,000 658,000,000 658,000,000  
Principal 1,020,000,000 $ 1,020,000,000 1,020,000,000 $ 1,020,000,000
AFG [Member]        
Long-term Debt, Fiscal Year Maturity [Abstract]        
Revolving credit line term   5 years    
Revolving credit line 500,000,000 $ 500,000,000 500,000,000  
Amount borrowed under AFG revolving credit facility $ 0 $ 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]        
Interest rate description for revolving credit facility     three-month LIBOR plus 0.875% (currently 1.51%)  
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%      
Interest rate swaps [Member] | Fair Value Hedge [Member] | Designated as Hedging Instrument [Member] | AFG [Member]        
Debt Instrument [Line Items]        
Effective interest rate, percentage 8.7515% 8.7515% 8.7515% 8.611%
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% 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 $ 9,000,000 $ 9,000,000 $ 9,000,000 $ 2,000,000
Senior Notes [Member] | AFG [Member]        
Long-term Debt, Fiscal Year Maturity [Abstract]        
Principal $ 708,000,000 $ 708,000,000 $ 708,000,000 708,000,000
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% 9.875%  
Senior Notes [Member] | 9-7/8% Senior Notes due June 2019 [Member] | AFG [Member]        
Long-term Debt, Fiscal Year Maturity [Abstract]        
Principal $ 350,000,000 $ 350,000,000 $ 350,000,000 $ 350,000,000
Stated interest rate, percentage 9.875% 9.875% 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]        
Principal $ 230,000,000 $ 230,000,000 $ 230,000,000 $ 230,000,000
Stated interest rate, percentage 6.375% 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]        
Principal $ 125,000,000 $ 125,000,000 $ 125,000,000 125,000,000
Stated interest rate, percentage 5.75% 5.75% 5.75%  
Subordinated Debt [Member] | AFG [Member]        
Long-term Debt, Fiscal Year Maturity [Abstract]        
Principal $ 300,000,000 $ 300,000,000 $ 300,000,000 300,000,000
Subordinated Debt [Member] | 6-1/4% Subordinated Debentures due September 2054 [Member] | AFG [Member]        
Long-term Debt, Fiscal Year Maturity [Abstract]        
Principal $ 150,000,000 $ 150,000,000 $ 150,000,000 150,000,000
Stated interest rate, percentage 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]        
Principal $ 150,000,000 $ 150,000,000 $ 150,000,000 150,000,000
Stated interest rate, percentage 6.00% 6.00% 6.00%  
Revolving Credit Facility [Member] | National Interstate [Member]        
Long-term Debt, Fiscal Year Maturity [Abstract]        
Revolving credit line $ 100,000,000 $ 100,000,000 $ 100,000,000  
Principal $ 12,000,000 $ 12,000,000 $ 12,000,000 $ 12,000,000
Revolving Credit Facility [Member] | LIBOR [Member] | National Interstate [Member]        
Debt Instrument [Line Items]        
Effective interest rate, percentage 1.51% 1.51% 1.51%  
Revolving Credit Facility [Member] | LIBOR [Member] | National Interstate [Member] | Maximum [Member]        
Long-term Debt, Fiscal Year Maturity [Abstract]        
Interest rate on revolving debt facility 0.875%