MARKETABLE SECURITIES (Tables)

3 Months Ended

Mar. 31, 2018

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 March 31, 2018 and December 31, 2017 were as follows (in thousands):

	March 31, 2018
Short-term marketable securities:		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial paper		$	227,630	$	1	$	(12	)	$	227,619
Corporate bonds		86,977		—		(282		)	86,695
Agency bonds		69,036		—		(70		)	68,966
U.S. government bonds		38,640		—		(31		)	38,609
Total short-term marketable securities		422,283		1		(395		)	421,889
Long-term marketable securities:
Agency bonds		$	11,912	$	—	$	(17	)	$	11,895
Corporate bonds		2,986		—		(9		)	2,977
Total long-term marketable securities		$	14,898	$	—	$	(26	)	$	14,872
Total marketable securities		$	437,181	$	1	$	(421	)	$	436,761

	December 31, 2017
Short-term marketable securities:	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Commercial paper	$	138,412	$	1	$	(1	)	$	138,412
Corporate bonds	45,006		—		(41		)	44,965
Agency bonds	78,958		—		(45		)	78,913
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 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 March 31, 2018 and December 31, 2017, aggregated by investment category and the length of time that the individual securities have been in a continuous loss position (in thousands):

	March 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	$	89,672	$	(291	)	$	—	$	—	$	89,672	$	(291	)
Agency bonds	80,861		(87		)	—		—		80,861		(87		)
U.S. government bonds	38,609		(31		)	—		—		38,609		(31		)
Commercial paper	18,866		(12		)	—		—		18,866		(12		)
Total	$	228,008	$	(421	)	$	—	$	—	$	228,008	$	(421	)

	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	)