Fair Value Measurements - Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	$ 4,186	$ 4,121
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	4,164	4,107
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	22	14
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Cash and cash equivalents | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	3,991	3,925
Cash and cash equivalents | Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	34	32
Cash and cash equivalents | 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	3,991	3,925
Cash and cash equivalents | 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	34	32
Cash and cash equivalents | 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 | 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 | 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 | 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 | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	139	150
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	21	13
Other current assets | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	1	1
Other current assets | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	139	150
Other current assets | 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
Other current assets | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Other current assets | Significant Other Observable Inputs (Level 2) | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Other current assets | 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	21	13
Other current assets | Significant Other Observable Inputs (Level 2) | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	1	1
Other current assets | Significant Unobservable Inputs (Level 3) | U.S. treasuries and government agency securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
Other current assets | 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
Other current assets | Significant Unobservable Inputs (Level 3) | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets at fair value	0	0
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		(1)
Accrued expenses and other liabilities | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities at fair value	(3)
Accrued expenses and other liabilities | 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 | Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign currency forward contracts | Not 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 | 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		(1)
Accrued expenses and other liabilities | Significant Other Observable Inputs (Level 2) | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities at fair value	(3)
Accrued expenses and other liabilities | 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
Accrued expenses and other liabilities | Significant Unobservable Inputs (Level 3) | Foreign currency forward contracts | Not Designated as Hedging Instrument
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities at fair value	$ 0