MARKETABLE SECURITIES (Tables)

9 Months Ended

Sep. 30, 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, all of which mature within one year, as of September 30, 2017 and December 31, 2016 were as follows (in thousands):

	September 30, 2017
Short-term marketable securities:	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Agency bonds	$	128,005	$	1	$	(53	)	$	127,953
Commercial paper	41,817		1		—			41,818
Agency discount notes	15,948		1		—			15,949
Corporate bonds	9,998		—		(5		)	9,993
Total marketable securities	$	195,768	$	3	$	(58	)	$	195,713
	December 31, 2016
Short-term marketable securities:	Amortized Cost		Gross Unrealized Gains		Gross Unrealized 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 September 30, 2017 and December 31, 2016, aggregated by investment category and the length of time that the individual securities have been in a continuous loss position (in thousands):

	September 30, 2017
	Less Than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Agency bonds	$	122,948	$	(53	)	$	—	$	—	$	122,948	$	(53	)
Corporate bonds	9,993		(5		)	—		—		9,993		(5		)
Total	$	132,941	$	(58	)	$	—	$	—	$	132,941	$	(58	)
	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	)