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Long-Term Debt - Narrative (Details) - USD ($)
9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Long-term Debt, Fiscal Year Maturity [Abstract]    
Scheduled principal payments on debt in next year $ 0  
Scheduled principal payments on debt in year two 0  
Scheduled principal payments on debt in year three 0  
Scheduled principal payments on debt in year four 0  
Scheduled principal payments on debt in year five 0  
Scheduled principal payments on debt in year six 0  
AFG    
Long-term Debt, Fiscal Year Maturity [Abstract]    
Revolving credit line 500,000,000  
Amount borrowed under revolving credit facility $ 0 $ 0
Interest rate description for revolving credit facility 1.00% to 1.875% (currently 1.375%) over LIBOR  
LIBOR | AFG    
Long-term Debt, Fiscal Year Maturity [Abstract]    
Interest rate on revolving debt facility 1.375%  
LIBOR | AFG | Minimum    
Long-term Debt, Fiscal Year Maturity [Abstract]    
Interest rate on revolving debt facility 1.00%  
LIBOR | AFG | Maximum    
Long-term Debt, Fiscal Year Maturity [Abstract]    
Interest rate on revolving debt facility 1.875%  
Senior Notes | 4.50% Senior Notes due June 2047 | AFG    
Long-term Debt, Fiscal Year Maturity [Abstract]    
Interest rate on debt instruments 4.50%  
Senior Notes | 3.50% Senior Notes due August 2026 | AFG    
Long-term Debt, Fiscal Year Maturity [Abstract]    
Interest rate on debt instruments 3.50%  
Subordinated Debentures | 6-1/4% Subordinated Debentures due September 2054 | AFG    
Long-term Debt, Fiscal Year Maturity [Abstract]    
Interest rate on debt instruments 6.25%  
Subordinated Debentures | 6% Subordinated Debentures due November 2055 | AFG    
Long-term Debt, Fiscal Year Maturity [Abstract]    
Interest rate on debt instruments 6.00%