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Fair Value Measurements - Narrative (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
stock
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Sep. 30, 2017
USD ($)
stock
Sep. 30, 2018
USD ($)
stock
professional
Sep. 30, 2017
USD ($)
stock
Dec. 31, 2017
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
AFG's internal investment professionals | professional         25    
Number of stocks transferred from Level 1 to Level 2 | stock 0            
Number of stocks transferred from Level 2 to Level 1 | stock 0            
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         68.00%    
Level 3 assets that were priced using non-binding broker quotes         $ 2,180    
Fair value of equity securities transferred into Level 3 $ 13     $ 188 100 $ 277  
Life contingent annuities 229       $ 229   $ 206
Maximum              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Percentage of internally developed Level 3 asset fair values to Shareholders' equity (are less than)         18.00%    
Fixed-indexed and variable-indexed annuities (embedded derivative), majority of future years | Minimum              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Unobservable input surrenders used in Level 3 fair value determination         11.00%    
Fixed-indexed and variable-indexed annuities (embedded derivative)              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Fair value of derivatives in annuity benefits accumulated measured using a discounted cash flow approach 3,110       $ 3,110    
Fixed-indexed and variable-indexed annuities (embedded derivative) | Minimum              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Unobservable input surrenders used in Level 3 fair value determination         3.00%    
Fixed-indexed and variable-indexed annuities (embedded derivative) | Maximum              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Unobservable input surrenders used in Level 3 fair value determination         23.00%    
Equity securities              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Fair value of equity securities transferred into Level 3 0   $ 29 $ 0 $ 30 $ 0  
Preferred stocks              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Number of stocks transferred from Level 1 to Level 2 | stock         2    
Fair Value of assets transferred from Level 1 to Level 2, during the first nine months $ 6       $ 6    
Number of stocks transferred from Level 2 to Level 1 | stock       1   3  
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   $ 17  
Not Designated as Hedging Instrument | Annuity benefits | Fixed-indexed annuities (embedded derivative)              
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]              
Total realized/unrealized gains (losses) included in net income for the embedded derivatives related to the unlocking of actuarial assumptions   $ (44)     $ (44)