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Fair Value Measurements - Narrative (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
USD ($)
stock
Sep. 30, 2016
USD ($)
stock
Sep. 30, 2017
USD ($)
stock
professional
Sep. 30, 2016
USD ($)
stock
Dec. 31, 2016
USD ($)
Fair Value Measurements (Textual) [Abstract]          
AFG's internal investment professionals | professional     25    
Level 3 assets as a percentage of total assets measured at fair value     5.00%    
Percentage of level 3 assets that were priced using non-binding broker quotes     76.00%    
Level 3 assets that were priced using non-binding broker quotes     $ 1,610    
Life contingent annuities $ 207   $ 207   $ 204
Maximum          
Fair Value Measurements (Textual) [Abstract]          
Percentage of internally developed Level 3 asset fair values to Shareholders' equity (are less than)     10.00%    
Fixed-indexed annuties (embedded derivative), majority of future years | Minimum          
Fair Value Measurements (Textual) [Abstract]          
Unobservable input surrenders used in Level 3 fair value determination     6.00%    
Fixed-indexed annuties (embedded derivative), majority of future years | Maximum          
Fair Value Measurements (Textual) [Abstract]          
Unobservable input surrenders used in Level 3 fair value determination     10.00%    
Fixed-indexed annuities (embedded derivative)          
Fair Value Measurements (Textual) [Abstract]          
Fair value of derivatives in annuity benefits accumulated measured using a discounted cash flow approach $ 2,290   $ 2,290    
Fixed-indexed annuities (embedded derivative) | Minimum          
Fair Value Measurements (Textual) [Abstract]          
Unobservable input surrenders used in Level 3 fair value determination     3.00%    
Fixed-indexed annuities (embedded derivative) | Maximum          
Fair Value Measurements (Textual) [Abstract]          
Unobservable input surrenders used in Level 3 fair value determination     23.00%    
Common stocks          
Fair Value Measurements (Textual) [Abstract]          
Number of stocks transferred from Level 1 to Level 2 | stock   1   1  
Fair value of assets transferred from Level 1 to Level 2, during the third quarter   $ 1      
Fair Value of assets transferred from Level 1 to Level 2, during the first nine months   1   $ 1  
Perpetual preferred stocks          
Fair Value Measurements (Textual) [Abstract]          
Number of stocks transferred from Level 2 to Level 1 | stock 1   3 6  
Fair value of assets transferred from Level 2 to Level 1, during the third quarter $ 1        
Fair Value of assets transferred from Level 2 to Level 1, during the first nine months $ 17 35 $ 17 $ 35  
Number of stocks transferred from Level 1 to Level 2 | stock       5  
Fair Value of assets transferred from Level 1 to Level 2, during the first nine months   $ 12   $ 12