Fair Value Measurements - Recurring Basis (Details) - Recurring - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	$ 2,987,000,000	$ 2,990,000,000
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	2,923,000,000	2,921,000,000
Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	43,000,000	38,000,000
Foreign currency forward contracts | Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	12,000,000	22,000,000
Total liabilities at fair value	(1,000,000)
ARS
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	9,000,000	9,000,000
Fair value measurements using quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	2,966,000,000	2,959,000,000
Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	2,923,000,000	2,921,000,000
Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	43,000,000	38,000,000
Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Foreign currency forward contracts | Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Total liabilities at fair value	0
Fair value measurements using quoted prices in active markets for identical assets (Level 1) | ARS
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	12,000,000	22,000,000
Fair value measurements using significant other observable inputs (Level 2) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant other observable inputs (Level 2) | Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant other observable inputs (Level 2) | Foreign currency forward contracts | Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	12,000,000	22,000,000
Total liabilities at fair value	(1,000,000)
Fair value measurements using significant other observable inputs (Level 2) | ARS
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	9,000,000	9,000,000
Fair value measurements using significant unobservable inputs (Level 3) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant unobservable inputs (Level 3) | Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Fair value measurements using significant unobservable inputs (Level 3) | Foreign currency forward contracts | Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Total liabilities at fair value	0
Fair value measurements using significant unobservable inputs (Level 3) | ARS
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	$ 9,000,000	$ 9,000,000