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Long-Term Debt - Narrative (Details) - USD ($)
9 Months Ended
Sep. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Long-term Debt, Fiscal Year Maturity [Abstract]      
Scheduled principal payments on debt remainder of fiscal 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%    
Subordinated Debentures | 5.875% Subordinated Debentures due in March 2059 | AFG      
Long-term Debt, Fiscal Year Maturity [Abstract]      
Subordinated Debentures, aggregate principal amount   $ 125,000,000  
Interest rate on debt instruments 5.875%