Financial Instruments - Schedule of Cash, Cash Equivalents, Restricted Cash and Marketable Securities (Details) - USD ($) $ in Millions		Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and Cash Equivalents		$ 901	$ 655
Short-Term Marketable Securities		65	15
Cash	[1],[2]	870	522
Cash and Restricted Cash	[1],[2]	870	522
Cash equivalents	[2]	870	522
Cash, cash equivalents, restricted cash and marketable securities, amortized cost		966	670
Cash, cash equivalents, restricted cash and marketable securities, fair value		966	670
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		31	128
Level 1 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and Cash Equivalents		31	128
Cash, cash equivalents, restricted cash and marketable securities, fair value		31	128
Level 2 | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		65	20
Fair Value	[1]	65	20
Cash and Cash Equivalents			5
Short-Term Marketable Securities		$ 65	$ 15

[1] As of June 30, 2019 and December 31, 2018, any unrealized gains or losses related to our marketable securities were not material.

[2] As of June 30, 2019 and December 31, 2018, 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.