Financial Instruments - Schedule of Cash, Cash Equivalents, Restricted Cash and Marketable Securities (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and Cash Equivalents		$ 635	$ 673
Short-Term Marketable Securities		15	35
Long-Term Marketable Securities		5	27
Cash	[1]	340	663
Cash equivalents	[1]	340	663
Cash, cash equivalents, restricted cash and marketable securities, amortized cost		655	735
Cash, cash equivalents, restricted cash and marketable securities, fair value		655	735
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		295	1
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		20	71
Fair Value		20	71
Cash and Cash Equivalents			9
Short-Term Marketable Securities		15	35
Long-Term Marketable Securities		5	27
Level 2 | U.S. Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		4	11
Fair Value		4	11
Short-Term Marketable Securities		4	6
Long-Term Marketable Securities			5
Level 2 | US treasury securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost			1
Fair Value			1
Short-Term Marketable Securities			1
Level 2 | Certificates of Deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		2	2
Fair Value		2	2
Short-Term Marketable Securities		2	2
Level 2 | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost			11
Fair Value			11
Cash and Cash Equivalents			9
Short-Term Marketable Securities			2
Level 2 | Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		14	46
Fair Value		14	46
Short-Term Marketable Securities		9	24
Long-Term Marketable Securities		$ 5	$ 22

[1] As of March 31, 2018 and December 31, 2017, our restricted cash which primarily consists of escrowed security deposits, was not material and is included in other long-term assets on our unaudited condensed consolidated balance sheets.