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Investments - Available-For-Sale Securities (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
One year or less (including restricted cash of $13) $ 2,969  
One year through two years 1,069  
Two years through three years 450  
Three years through four years 264  
Estimated Fair Value 4,752  
Restricted cash 13  
Short-term Investments    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 2,970 $ 2,722
Gross Unrealized Gains 1 0
Gross Unrealized Losses (2) (9)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value 2,969 2,713
Short-term Investments | Restricted cash    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 13 17
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value 13 17
Short-term Investments | Corporate debt securities    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 2,957 2,615
Gross Unrealized Gains 1 0
Gross Unrealized Losses (2) (9)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value 2,956 2,606
Short-term Investments | Government and agency securities    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost   90
Gross Unrealized Gains   0
Gross Unrealized Losses   0
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value   90
Long-term Investments    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 1,782 3,682
Gross Unrealized Gains 4 1
Gross Unrealized Losses (3) (48)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value 1,783 3,635
Long-term Investments | Corporate debt securities    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 1,782 3,682
Gross Unrealized Gains 4 1
Gross Unrealized Losses (3) (48)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value $ 1,783 $ 3,635