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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
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	92,798	116,042
Gross Unrealized Gains	4	54
Gross Unrealized Losses	(210)	(107)
Estimated Fair Value	92,592	115,989
Money market funds
Included in cash and cash equivalents:
Amortized Cost	11,030	11,193
Estimated Fair Value	11,030	11,193
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	12,500	5,000
Gross Unrealized Losses	(40)	(3)
Estimated Fair Value	12,460	4,997
Government-sponsored enterprise securities (due in one to two years)
Marketable securities:
Amortized Cost		12,500
Gross Unrealized Losses		(42)
Estimated Fair Value		12,458
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	10,928	31,024
Gross Unrealized Gains	4	50
Gross Unrealized Losses	(1)	(5)
Estimated Fair Value	10,931	31,069
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	55,067	66,012
Gross Unrealized Gains		4
Gross Unrealized Losses	(107)	(47)
Estimated Fair Value	54,960	65,969
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	14,303	1,506
Gross Unrealized Losses	(62)	(10)
Estimated Fair Value	$ 14,241	$ 1,496

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