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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Restricted cash:
Amortized Cost	$ 270	$ 269
Marketable securities:
Amortized Cost	143,994	171,479
Gross Unrealized Gains	288	29
Gross Unrealized Losses	(7)	(212)
Estimated Fair Value	144,275	171,296
Money market funds
Included in cash and cash equivalents:
Amortized Cost	15,457	7,003
Estimated Fair Value	15,457	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	42,000	57,594
Gross Unrealized Gains	83	22
Gross Unrealized Losses	(1)	(29)
Estimated Fair Value	42,082	57,587
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	92,546	95,238
Gross Unrealized Gains	155	7
Gross Unrealized Losses	(5)	(118)
Estimated Fair Value	92,696	95,127
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	9,448	18,647
Gross Unrealized Gains	50
Gross Unrealized Losses	(1)	(65)
Estimated Fair Value	$ 9,497	$ 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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