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Fair Value Measurements - Narrative (Details)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
stock
Mar. 31, 2018
USD ($)
Jun. 30, 2017
USD ($)
stock
Jun. 30, 2018
USD ($)
stock
professional
Jun. 30, 2017
USD ($)
stock
Dec. 31, 2017
USD ($)
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       6.00%    
Percentage of level 3 assets that were priced using non-binding broker quotes       73.00%    
Level 3 assets that were priced using non-binding broker quotes       $ 2,140    
Fair value of equity securities transferred into Level 3 $ 51   $ 32 87 $ 89  
Life contingent annuities 213     $ 213   $ 206
Maximum            
Fair Value Measurements (Textual) [Abstract]            
Percentage of internally developed Level 3 asset fair values to Shareholders' equity (are less than)       14.00%    
Fixed-indexed and variable-indexed annuities (embedded derivative), majority of future years | Minimum            
Fair Value Measurements (Textual) [Abstract]            
Unobservable input surrenders used in Level 3 fair value determination       7.00%    
Fixed-indexed and variable-indexed annuities (embedded derivative), majority of future years | Maximum            
Fair Value Measurements (Textual) [Abstract]            
Unobservable input surrenders used in Level 3 fair value determination       11.00%    
Fixed-indexed and variable-indexed annuities (embedded derivative)            
Fair Value Measurements (Textual) [Abstract]            
Fair value of derivatives in annuity benefits accumulated measured using a discounted cash flow approach 2,780     $ 2,780    
Fixed-indexed and variable-indexed annuities (embedded derivative) | Minimum            
Fair Value Measurements (Textual) [Abstract]            
Unobservable input surrenders used in Level 3 fair value determination       3.00%    
Fixed-indexed and variable-indexed annuities (embedded derivative) | Maximum            
Fair Value Measurements (Textual) [Abstract]            
Unobservable input surrenders used in Level 3 fair value determination       23.00%    
Equity securities            
Fair Value Measurements (Textual) [Abstract]            
Fair value of equity securities transferred into Level 3 $ 1 $ 29 $ 0 $ 30 $ 0  
Preferred stocks            
Fair Value Measurements (Textual) [Abstract]            
Number of stocks transferred from Level 1 to Level 2 | stock 2     2    
Fair Value of assets transferred from Level 1 to Level 2, during the second quarter $ 6          
Fair Value of assets transferred from Level 1 to Level 2, during the first six months $ 6     $ 6    
Number of stocks transferred from Level 2 to Level 1 | stock     2   2  
Fair Value of assets transferred from Level 2 to Level 1, during the second quarter     $ 16      
Fair Value of assets transferred from Level 2 to Level 1, during the first six months     $ 16   $ 16