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Fair Value Measurements - Narrative (Details)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
basispoint
security
professional
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
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   7.00%    
Percentage of level 3 assets that were priced using non-binding broker quotes   69.00%    
Level 3 assets that were priced using non-binding broker quotes   $ 2,410    
Percent of spreads between 400 and 700 basis points   80.00%    
Fair value of equity securities transferred into level 3   $ 270 $ 289 $ 139
Life contingent annuities   232 206  
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   $ 2,720    
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%    
Fixed-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 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%    
Not Designated as Hedging Instrument | Annuity benefits | Fixed-indexed and variable-indexed annuities (embedded derivative)        
Fair Value Measurements (Textual) [Abstract]        
Total realized/unrealized gains (losses) included in net income for the embedded derivatives related to the unlocking of actuarial assumptions   $ (44) (25) (17)
Level 3        
Fair Value Measurements (Textual) [Abstract]        
Internally developed level 3 assets   1,050    
Fixed maturities        
Fair Value Measurements (Textual) [Abstract]        
Fair value of equity securities transferred into level 3   240 289 124
Equity securities        
Fair Value Measurements (Textual) [Abstract]        
Fair value of equity securities transferred into level 3 $ 29 30 $ 0 $ 15
Third party pricing model | Fixed maturities | Level 3        
Fair Value Measurements (Textual) [Abstract]        
Internally developed level 3 assets   $ 590    
Securities priced using third-party model | security   100    
Basis points credit spread applied by management using third party model | basispoint   585    
Third party pricing model | Fixed maturities | Level 3 | Minimum        
Fair Value Measurements (Textual) [Abstract]        
Basis points credit spread applied by management using third party model | basispoint   131    
Third party pricing model | Fixed maturities | Level 3 | Maximum        
Fair Value Measurements (Textual) [Abstract]        
Basis points credit spread applied by management using third party model | basispoint   2,966    
Maximum | Third party pricing model | Fixed maturities | Level 3        
Fair Value Measurements (Textual) [Abstract]        
Basis points credit spread applied by management using third party model | basispoint   700    
Minimum | Third party pricing model | Fixed maturities | Level 3        
Fair Value Measurements (Textual) [Abstract]        
Basis points credit spread applied by management using third party model | basispoint   400