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Investments Held-to-maturity securities in a continuous unrealized loss position (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]    
Fair value, less than 12 months $ 531 $ 867
Unrealized losses, less than 12 months (1) (4)
Fair value, 12 months or more 57 0
Unrealized losses, 12 months or more 0 0
Total fair value 588 867
Total unrealized losses (1) (4)
Corporate And Other Debt Securities [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Fair value, less than 12 months 263 473
Unrealized losses, less than 12 months 0 (2)
Fair value, 12 months or more 57 0
Unrealized losses, 12 months or more 0 0
Total fair value 320 473
Total unrealized losses 0 (2)
Commercial Paper [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Fair value, less than 12 months 268 394
Unrealized losses, less than 12 months (1) (2)
Fair value, 12 months or more 0 0
Unrealized losses, 12 months or more 0 0
Total fair value 268 394
Total unrealized losses $ (1) $ (2)