MARKETABLE DEBT SECURITIES (Tables)

9 Months Ended

Sep. 30, 2018

Investments In Marketable Debt Securities [Abstract]

Summary of Company's Marketable Debt Securities

The following tables summarize our marketable debt securities at September 30, 2018 and December 31, 2017, in thousands:

	At September 30, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	18,800	$	—	$	—		$	18,800
Commercial paper		101,379		—		(3	)		101,376
Corporate notes		324,615		11		(247	)		324,379
U.S. government-sponsored enterprise securities		111,427		—		(88	)		111,339
U.S. treasury securities		349,601		1		(274	)		349,328
Total	$	905,822	$	12	$	(612	)	$	905,222

	At December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Certificates of deposit	$	30,200	$	—	$	—		$	30,200
Commercial paper		56,951		—		—			56,951
Corporate notes		373,736		11		(495	)		373,252
U.S. government-sponsored enterprise securities		399,281		—		(983	)		398,298
U.S. treasury securities		200,649		1		(175	)		200,475
Total	$	1,060,817	$	12	$	(1,653	)	$	1,059,176

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

The following table summarizes our available-for-sale debt securities by contractual maturity, at September 30, 2018, in thousands:

	At September 30, 2018
	AmortizedCost		Fair Value
Less than one year	$	905,822	$	905,222
Greater than one year but less than two years		—		—
Total	$	905,822	$	905,222