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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Restricted cash:
Amortized Cost	$ 268	$ 268
Marketable securities:
Amortized Cost	102,603	116,042
Gross Unrealized Gains	28	54
Gross Unrealized Losses	(80)	(107)
Estimated Fair Value	102,551	115,989
Money market funds
Included in cash and cash equivalents:
Amortized Cost	8,133	11,193
Estimated Fair Value	8,133	11,193
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	17,500	5,000
Gross Unrealized Losses	(30)	(3)
Estimated Fair Value	17,470	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	11,159	31,024
Gross Unrealized Gains	25	50
Gross Unrealized Losses		(5)
Estimated Fair Value	11,184	31,069
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	58,353	66,012
Gross Unrealized Gains	3	4
Gross Unrealized Losses	(19)	(47)
Estimated Fair Value	58,337	65,969
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	15,591	1,506
Gross Unrealized Losses	(31)	(10)
Estimated Fair Value	$ 15,560	$ 1,496

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