MARKETABLE DEBT SECURITIES (Tables)

3 Months Ended

Mar. 31, 2019

Investments Debt And Equity Securities [Abstract]

Summary of Company's Marketable Debt Securities

The following tables summarize our marketable debt securities at March 31, 2019 and December 31, 2018, in thousands:

	At March 31, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	3,000	$	—	$	—		$	3,000
Commercial paper		34,394		—		—			34,394
Corporate notes		113,281		1		(29	)		113,253
U.S. government-sponsored enterprise securities		2,496		—		—			2,496
U.S. treasury securities		896,699		25		(58	)		896,666
Total	$	1,049,870	$	26	$	(87	)	$	1,049,809

	At December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	8,951	$	—	$	—		$	8,951
Commercial paper		57,197		—		—			57,197
Corporate notes		232,695		—		(285	)		232,410
U.S. government-sponsored enterprise securities		39,031		—		(13	)		39,018
U.S. treasury securities		546,631		1		(124	)		546,508
Total	$	884,505	$	1	$	(422	)	$	884,084

Summary of Fair Value of Marketable Debt Securities

The fair values of our marketable debt securities by classification in the condensed consolidated balance sheets were as follows, in thousands:

	At March 31, 2019		At December 31, 2018
Cash and cash equivalents	$	530,231	$	221,281
Marketable debt securities		519,578		662,803
Total	$	1,049,809	$	884,084

Summary of Available-For-Sale Debt Securities by Contractual Maturity

The following table summarizes our available-for-sale debt securities by contractual maturity, at March 31, 2019, in thousands:

	At March 31, 2019
	AmortizedCost		Fair Value
Less than one year	$	1,049,870	$	1,049,809
Greater than one year but less than two years		—		—
Total	$	1,049,870	$	1,049,809