FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Included in cash and cash equivalents:
Amortized Cost		$ 15,022
Gross Unrealized Losses		(1)
Estimated Fair Value		15,021
Restricted cash:
Amortized Cost	$ 269	268
Marketable securities:
Amortized Cost	171,877	92,798
Gross Unrealized Gains	64	4
Gross Unrealized Losses	(176)	(210)
Estimated Fair Value	171,765	92,592
Money market funds
Included in cash and cash equivalents:
Amortized Cost	10,065	11,030
Estimated Fair Value	10,065	11,030
Corporate notes
Included in cash and cash equivalents:
Amortized Cost		1,750
Gross Unrealized Losses		(1)
Estimated Fair Value		1,749
Certificate of deposit
Restricted cash:
Amortized Cost	269	268
Estimated Fair Value	269	268
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	63,019	10,928
Gross Unrealized Gains	62	4
Gross Unrealized Losses	(22)	(1)
Estimated Fair Value	63,059	10,931
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	90,682	55,067
Gross Unrealized Gains	2
Gross Unrealized Losses	(121)	(107)
Estimated Fair Value	90,563	54,960
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	18,176	14,303
Gross Unrealized Losses	(33)	(62)
Estimated Fair Value	$ 18,143	14,241
Commercial paper
Included in cash and cash equivalents:
Amortized Cost		2,242
Estimated Fair Value		2,242
Government-sponsored enterprise securities (due in less than one year)
Marketable securities:
Amortized Cost		12,500
Gross Unrealized Losses		(40)
Estimated Fair Value		$ 12,460

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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