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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Included in cash and cash equivalents:
Amortized Cost	$ 17,139	$ 15,022
Gross Unrealized Gains	1
Gross Unrealized Losses	(1)	(1)
Estimated Fair Value	17,139	15,021
Restricted cash:
Amortized Cost	269	268
Marketable securities:
Amortized Cost	162,127	92,798
Gross Unrealized Gains	181	4
Gross Unrealized Losses	(258)	(210)
Estimated Fair Value	162,050	92,592
Money market funds
Included in cash and cash equivalents:
Amortized Cost	11,450	11,030
Estimated Fair Value	11,450	11,030
Commercial paper
Included in cash and cash equivalents:
Amortized Cost	5,689	2,242
Gross Unrealized Gains	1
Gross Unrealized Losses	(1)
Estimated Fair Value	5,689	2,242
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
Government-sponsored enterprise securities (due in less than one year)
Marketable securities:
Amortized Cost	10,000	12,500
Gross Unrealized Losses	(10)	(40)
Estimated Fair Value	9,990	12,460
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	56,917	10,928
Gross Unrealized Gains	181	4
Gross Unrealized Losses	(4)	(1)
Estimated Fair Value	57,094	10,931
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	74,930	55,067
Gross Unrealized Losses	(214)	(107)
Estimated Fair Value	74,716	54,960
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	20,280	14,303
Gross Unrealized Losses	(30)	(62)
Estimated Fair Value	$ 20,250	$ 14,241

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