MARKETABLE SECURITIES (Tables)

12 Months Ended

Dec. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Schedule of the Fair Value to Amortized Cost Basis of Securities Held-to-Maturity

The amortized cost, gross unrealized gains and losses, and fair value of securities held-to-maturity as of December 31, 2018 and 2017 were as follows (in thousands):

	As of December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term marketable securities:
Commercial paper	$	175,070	$	—	$	—		$	175,070
Corporate bonds	131,626		8		(138		)	131,496
U.S. government bonds	65,513		—		(11		)	65,502
Agency bonds	50,887		—		(41		)	50,846
Total marketable securities	$	423,096	$	8	$	(190	)	$	422,914
	As of December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term marketable securities:
Commercial paper	$	138,412	$	1	$	(1	)	$	138,412
Agency bonds	78,958		—		(45		)	78,913
Corporate bonds	45,006		—		(41		)	44,965
Agency discount bonds	10,990		—		(1		)	10,989
Total short-term marketable securities	$	273,366	$	1	$	(88	)	$	273,279
Long-term marketable securities:
Corporate bonds	$	25,032	$	—	$	(71	)	$	24,961
Total long-term marketable securities	$	25,032	$	—	$	(71	)	$	24,961
Total marketable securities	$	298,398	$	1	$	(159	)	$	298,240

Schedule of Unrealized Loss on Investments

The following tables present gross unrealized losses and fair values for those securities that were in an unrealized loss position as of December 31, 2018 and 2017, aggregated by investment category and the length of time that the individual securities have been in a continuous loss position (in thousands):

	As of December 31, 2018
	Less Than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Corporate bonds	$	121,566	$	(138	)	$	—	$	—	$	121,566	$	(138	)
U.S. government bonds	65,502		(11		)	—		—		65,502		(11		)
Agency bonds	50,846		(41		)	—		—		50,846		(41		)
Total	$	237,914	$	(190	)	$	—	$	—	$	237,914	$	(190	)
	As of December 31, 2017
	Less Than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Agency bonds	$	78,913	$	(45	)	$	—	$	—	$	78,913	$	(45	)
Corporate bonds	62,927		(112		)	—		—		62,927		(112		)
Agency discount notes	10,989		(1		)	—		—		10,989		(1		)
Commercial paper	3,975		(1		)	—		—		3,975		(1		)
Total	$	156,804	$	(159	)	$	—	$	—	$	156,804	$	(159	)