Fair Value Measurements - Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	$ 3,484	$ 2,990
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	3,484	2,959
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	0	22
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	0	9
Cash and cash equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	3,355	2,921
Cash and cash equivalents | Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	49	38
Cash and cash equivalents | 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	3,355	2,921
Cash and cash equivalents | 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	49	38
Cash and cash equivalents | 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
Cash and cash equivalents | 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
Cash and cash equivalents | 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
Cash and cash equivalents | 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
Other current assets | US treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	80
Other current assets | Foreign currency forward contracts | Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value		22
Other current assets | Fair value measurements using quoted prices in active markets for identical assets (Level 1) | US treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	80
Other current assets | 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
Other current assets | Fair value measurements using significant other observable inputs (Level 2) | US treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0
Other current assets | 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		22
Other current assets | Fair value measurements using significant unobservable inputs (Level 3) | US treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0
Other current assets | 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
Other assets | ARS
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value		9
Other assets | 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
Other assets | 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
Other assets | 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
Accrued expenses and other liabilities | Foreign currency forward contracts | Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities at fair value	(10)
Accrued expenses and other liabilities | 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 liabilities at fair value	0
Accrued expenses and other liabilities | 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 liabilities at fair value	(10)
Accrued expenses and other liabilities | 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 liabilities at fair value	$ 0