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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 268	268
Marketable securities:
Amortized Cost	95,383	92,798
Gross Unrealized Gains	7	4
Gross Unrealized Losses	(338)	(210)
Estimated Fair Value	95,052	92,592
Money market funds
Included in cash and cash equivalents:
Amortized Cost	5,383	11,030
Estimated Fair Value	5,383	11,030
Commercial paper
Included in cash and cash equivalents:
Amortized Cost		2,242
Estimated Fair Value		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	268	268
Estimated Fair Value	268	268
Government-sponsored enterprise securities (due in less than one year)
Marketable securities:
Amortized Cost	10,000	12,500
Gross Unrealized Losses	(30)	(40)
Estimated Fair Value	9,970	12,460
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	11,923	10,928
Gross Unrealized Gains	7	4
Gross Unrealized Losses	(5)	(1)
Estimated Fair Value	11,925	10,931
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	60,201	55,067
Gross Unrealized Losses	(228)	(107)
Estimated Fair Value	59,973	54,960
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	13,259	14,303
Gross Unrealized Losses	(75)	(62)
Estimated Fair Value	$ 13,184	$ 14,241

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