Financial Instruments - Schedule of Cash, Cash Equivalents, Restricted Cash and Marketable Securities (Details) - USD ($) $ in Millions		Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and Cash Equivalents		$ 838	$ 655
Short-Term Marketable Securities		95	15
Cash	[1],[2]	828	522
Cash and Restricted Cash	[2]	828	522
Cash equivalents	[2]	828	522
Cash, cash equivalents, restricted cash and marketable securities, amortized cost		933	670
Cash, cash equivalents, restricted cash and marketable securities, fair value	[1]	933	670
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents		10	128
Level 1 | Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and Cash Equivalents		10	128
Cash, cash equivalents, restricted cash and marketable securities, fair value	[1]	10	128
Level 2 | Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		95	20
Fair Value	[1]	95	20
Cash and Cash Equivalents			5
Short-Term Marketable Securities		$ 95	$ 15

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

[2] As of September 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.