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Investments Investments (Limited Partnership Investments) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Jan. 02, 2020
Dec. 31, 2019
Limited Partnership [Member] | Fair Value Measured at Net Asset Value Per Share [Member]            
Net Investment Income [Line Items]            
Investments $ 366   $ 366     $ 320
Net gains from limited partnership investments 63 $ 5 73 $ 29    
BeiGene [Member]            
Net Investment Income [Line Items]            
Carrying value of equity method investment $ 3,000   $ 3,000      
Equity Method Investment, Aggregate Cost         $ 2,800  
Ownership percentage 20.40%   20.40%   20.50%  
Equity Method Investments,Quoted Market Price $ 5,300   $ 5,300   $ 2,600  
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity         $ 2,400  
Equity method investment, change in carrying value 505   505      
Income (Loss) from Equity Method Investments (68)   (143)      
Equity Method Investment, Amortization of Difference Between Carrying Amount and Underlying Equity 36   72      
Equity Method Investment, Change In Carrying Value, Other $ 23   $ 23      
Minimum [Member] | BeiGene [Member]            
Net Investment Income [Line Items]            
Finite-Lived Intangible Asset, Useful Life     8 years      
Maximum [Member] | BeiGene [Member]            
Net Investment Income [Line Items]            
Finite-Lived Intangible Asset, Useful Life     15 years