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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Included in cash and cash equivalents:
Amortized Cost		$ 10,661
Estimated Fair Value		10,661
Restricted cash:
Amortized Cost	$ 270	270
Marketable securities:
Amortized Cost	112,975	145,200
Gross Unrealized Gains	105	157
Gross Unrealized Losses	(229)	(25)
Estimated Fair Value	112,851	145,332
Money market funds
Included in cash and cash equivalents:
Amortized Cost	18,370	6,671
Estimated Fair Value	18,370	6,671
Commercial paper
Included in cash and cash equivalents:
Amortized Cost		3,990
Estimated Fair Value		3,990
Certificate of deposit
Restricted cash:
Amortized Cost	270	270
Estimated Fair Value	270	270
Government-sponsored enterprise securities (due in less than one year)
Marketable securities:
Amortized Cost	11,504	6,506
Gross Unrealized Gains	16	6
Estimated Fair Value	11,520	6,512
Government-sponsored enterprise securities (due in one to two years)
Marketable securities:
Amortized Cost		6,999
Gross Unrealized Gains		1
Estimated Fair Value		7,000
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	29,028	40,110
Gross Unrealized Gains	37	33
Gross Unrealized Losses		(3)
Estimated Fair Value	29,065	40,140
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	71,422	78,926
Gross Unrealized Gains	44	116
Gross Unrealized Losses	(229)	(13)
Estimated Fair Value	71,237	79,029
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	1,021	12,659
Gross Unrealized Gains	8	1
Gross Unrealized Losses		(9)
Estimated Fair Value	$ 1,029	$ 12,651

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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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