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Fair Value Measurements - Narrative (Details)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
USD ($)
Dec. 31, 2019
USD ($)
basispoint
professional
security
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Fair Value Measurements (Textual) [Abstract]        
AFG's internal investment professionals | professional   20    
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   57.00%    
Level 3 assets that were priced using non-binding broker quotes   $ 2,170    
Percent of spreads between 200 and 700 basis points   70.00%    
Fair value of equity securities transferred into level 3   $ 244 $ 270 $ 289
Life contingent annuities   247 232  
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   $ 3,730    
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   22.00%    
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   10.00%    
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   $ 181 (44) (25)
Level 3        
Fair Value Measurements (Textual) [Abstract]        
Internally developed level 3 assets   1,360    
Fixed maturities        
Fair Value Measurements (Textual) [Abstract]        
Fair value of equity securities transferred into level 3   188 240 289
Equity securities        
Fair Value Measurements (Textual) [Abstract]        
Fair value of equity securities transferred into level 3 $ 29 56 $ 30 $ 0
Third party pricing model | Fixed maturities | Level 3        
Fair Value Measurements (Textual) [Abstract]        
Internally developed level 3 assets   $ 716    
Securities priced using third-party model | security   35    
Basis points credit spread applied by management using third party model | basispoint   396    
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   100    
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   1,253    
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   200