The following tables show our cash, cash equivalents, restricted cash and short-term and long-term available-for-sale marketable debt securities, by major security type, that are measured at fair value on a recurring basis and were categorized using the fair value hierarchy, as well as their classification on our unaudited condensed consolidated balance sheets, as of the periods presented (in millions):

	March 31, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value		Cash, Cash Equivalents and Restricted Cash		Short-Term Marketable Securities		Long-Term Marketable Securities
Cash and restricted cash (1)	$	340	$	—	$	—	$	340	$	340	$	—	$	—
Level 1:
Money market funds		295		—		—		295		295		—		—
Level 2:
U.S. agency securities		4		—		—		4		—		4		—
Certificates of deposit		2		—		—		2		—		2		—
Corporate debt securities		14		—		—		14		—		9		5
Subtotal		20		—		—		20		—		15		5
Total	$	655		—		—	$	655	$	635	$	15	$	5

	December 31, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value		Cash, Cash Equivalents and Restricted Cash		Short-Term Marketable Securities		Long-Term Marketable Securities
Cash and restricted cash (1)	$	663	$	—	$	—	$	663	$	663	$	—	$	—
Level 1:
Money market funds		1		—		—		1		1		—		—
Level 2:
U.S. agency securities		11		—		—		11		—		6		5
U.S. treasury securities		1		—		—		1		—		1		—
Certificates of deposit		2		—		—		2		—		2		—
Commercial paper		11		—		—		11		9		2		—
Corporate debt securities		46		—		—		46		—		24		22
Subtotal		71		—		—		71		9		35		27
Total	$	735		—		—	$	735	$	673		35		27

(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.