Fair Value Measurements (Details) - Recurring - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	$ 1,667	$ 4,524
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	1,613	4,475
Foreign government treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	34	40
Foreign currency forward contracts designated as hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	11
Liabilities measured at fair value on a recurring basis, gain loss included in earnings
Total liabilities at fair value	(4)
ARS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	9	9
Fair value measurements using quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	1,647	4,515
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 and Nonrecurring Basis [Line Items]
Total assets at fair value	1,613	4,475
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 and Nonrecurring Basis [Line Items]
Total assets at fair value	34	40
Fair value measurements using quoted prices in active markets for identical assets (Level 1) | Foreign currency forward contracts designated as hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0
Liabilities measured at fair value on a recurring basis, gain loss included in earnings
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 and Nonrecurring 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 and Nonrecurring Basis [Line Items]
Total assets at fair value	11	0
Fair value measurements using significant other observable inputs (Level 2) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring 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 and Nonrecurring 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 hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	11
Liabilities measured at fair value on a recurring basis, gain loss included in earnings
Total liabilities at fair value	(4)
Fair value measurements using significant other observable inputs (Level 2) | ARS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring 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 and Nonrecurring Basis [Line Items]
Total assets at fair value	9	9
Fair value measurements using significant unobservable inputs (Level 3) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring 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 and Nonrecurring 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 hedges
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	0
Liabilities measured at fair value on a recurring basis, gain loss included in earnings
Total liabilities at fair value	0
Fair value measurements using significant unobservable inputs (Level 3) | ARS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets at fair value	$ 9	$ 9