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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Restricted cash:
Amortized Cost	$ 270	$ 269
Marketable securities:
Amortized Cost	162,952	171,479
Gross Unrealized Gains	136	29
Gross Unrealized Losses	(37)	(212)
Estimated Fair Value	163,051	171,296
Money market funds
Included in cash and cash equivalents:
Amortized Cost	4,750	7,003
Estimated Fair Value	4,750	7,003
Certificate of deposit
Restricted cash:
Amortized Cost	270	269
Estimated Fair Value	270	269
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	56,474	57,594
Gross Unrealized Gains	45	22
Gross Unrealized Losses	(19)	(29)
Estimated Fair Value	56,500	57,587
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	88,648	95,238
Gross Unrealized Gains	45	7
Gross Unrealized Losses	(13)	(118)
Estimated Fair Value	88,680	95,127
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	17,830	18,647
Gross Unrealized Gains	46
Gross Unrealized Losses	(5)	(65)
Estimated Fair Value	$ 17,871	$ 18,582

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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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