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Fair Value of Assets and Liabilities - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Fair value of assets and liabilities measured on recurring and non-recurring basis    
Fair value $ 59,857 $ 59,044
Significant unobservable inputs (Level 3)    
Fair value of assets and liabilities measured on recurring and non-recurring basis    
Assets at fair value 680 738
Recurring | Significant unobservable inputs (Level 3)    
Fair value of assets and liabilities measured on recurring and non-recurring basis    
Fair value 285 342
Assets at fair value 677 738
Fixed Income Securities Valued Based on Nonbinding Broker Quotes    
Fair value of assets and liabilities measured on recurring and non-recurring basis    
Assets at fair value 37 50
Municipal Not Rated by Third Party Credit Rating Agencies    
Fair value of assets and liabilities measured on recurring and non-recurring basis    
Assets at fair value 33 $ 36
Limited partnership interests    
Fair value of assets and liabilities measured on recurring and non-recurring basis    
Commitments to invest in limited partnership interests $ 459  
Limited partnership interests | Minimum    
Fair value of assets and liabilities measured on recurring and non-recurring basis    
Investment assets, useful life 10 years  
Limited partnership interests | Maximum    
Fair value of assets and liabilities measured on recurring and non-recurring basis    
Investment assets, useful life 12 years