v3.10.0.1
Investments - Available-For-Sale Securities (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
One year or less (including restricted cash of $15) $ 2,752  
One year through two years 1,973  
Two years through three years 1,451  
Three years through four years 591  
Four years through five years 117  
Estimated Fair Value 6,884  
Restricted cash 15  
Short-term Investments    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 2,758 $ 3,746
Gross Unrealized Gains 1 1
Gross Unrealized Losses (7) (4)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value 2,752 3,743
Short-term Investments | Restricted cash    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 15 20
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value 15 20
Short-term Investments | Corporate debt securities    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 2,702 3,726
Gross Unrealized Gains 1 1
Gross Unrealized Losses (7) (4)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value 2,696 3,723
Short-term Investments | Government and agency securities    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 41  
Gross Unrealized Gains 0  
Gross Unrealized Losses 0  
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value 41  
Long-term Investments    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 4,171 5,458
Gross Unrealized Gains 7 12
Gross Unrealized Losses (46) (24)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value 4,132 5,446
Long-term Investments | Corporate debt securities    
Debt Securities, Available-for-sale, Unrealized Gain (Loss) [Abstract]    
Gross Amortized Cost 4,171 5,458
Gross Unrealized Gains 7 12
Gross Unrealized Losses (46) (24)
Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract]    
Estimated Fair Value $ 4,132 $ 5,446