MARKETABLE SECURITIES (Tables)

12 Months Ended

Dec. 31, 2017

Marketable 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, 2017 and 2016 were as follows (in thousands):

	As of December 31, 2017
			Gross		Gross
			Unrealized		Unrealized
Short-term marketable securities:	Amortized Cost		Gains		Losses			Fair Value
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
	As of December 31, 2016
			Gross		Gross
			Unrealized		Unrealized
Short-term marketable securities:	Amortized Cost		Gains		Losses			Fair Value
Agency bonds	$	152,429	$	18	$	(53	)	$	152,394
Commercial paper	45,894		—		—			45,894
Corporate bonds	9,009		—		(3		)	9,006
Total marketable securities	$	207,332	$	18	$	(56	)	$	207,294

Schedule of Securities in an Unrealized Loss Position

The following tables present gross unrealized losses and fair values for those securities that were in an unrealized loss position as of December 31, 2017 and 2016, 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, 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	)
	As of December 31, 2016
	Less Than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Agency bonds	$	92,018	$	(53	)	$	—	$	—	$	92,018	$	(53	)
Corporate bonds	8,006		(3		)	—		—		8,006		(3		)
Total	$	100,024	$	(56	)	$	—	$	—	$	100,024	$	(56	)