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Investments Investments (Limited Partnership Investments) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Limited Partnership [Member] | Fair Value Measured at Net Asset Value Per Share [Member]          
Net Investment Income [Line Items]          
Alternative investments $ 616   $ 616   $ 496
Net gains (losses) from limited partnership investments (43) $ (10) 165 $ 10  
BeiGene [Member]          
Net Investment Income [Line Items]          
Carrying value of equity method investment $ 2,800   $ 2,800    
Ownership percentage 20.30%   20.30%    
Equity Method Investments,Quoted Market Price $ 6,400   $ 6,400    
Income (Loss) from Equity Method Investments 14   (83)    
Equity Method Investment, Amortization of Difference Between Carrying Amount and Underlying Equity 42   84    
Equity Method Investment, Change In Carrying Value, Other $ 21   $ 38