FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Included in cash and cash equivalents:
Amortized Cost		$ 10,661
Estimated Fair Value		10,661
Restricted cash:
Amortized Cost	$ 363	270
Estimated Fair Value	363
Marketable securities:
Amortized Cost	228,265	145,200
Gross Unrealized Gains	183	157
Gross Unrealized Losses	(13)	(25)
Estimated Fair Value	228,435	145,332
Money market funds
Included in cash and cash equivalents:
Amortized Cost	41,608	6,671
Estimated Fair Value	41,608	6,671
Restricted cash:
Amortized Cost	92
Estimated Fair Value	92
Commercial paper
Included in cash and cash equivalents:
Amortized Cost		3,990
Estimated Fair Value		3,990
Certificate of deposit
Restricted cash:
Amortized Cost	271	270
Estimated Fair Value	271	270
U.S. Treasury securities (due in one to two years)
Marketable securities:
Amortized Cost	5,635
Gross Unrealized Gains	1
Estimated Fair Value	5,636
Government-sponsored enterprise securities (due in less than one year)
Marketable securities:
Amortized Cost	3,000	6,506
Gross Unrealized Gains		6
Estimated Fair Value	3,000	6,512
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	88,047	40,110
Gross Unrealized Gains	49	33
Gross Unrealized Losses	(2)	(3)
Estimated Fair Value	88,094	40,140
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	91,471	78,926
Gross Unrealized Gains	107	116
Gross Unrealized Losses	(5)	(13)
Estimated Fair Value	91,573	79,029
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	29,112	12,659
Gross Unrealized Gains	25	1
Gross Unrealized Losses	(5)	(9)
Estimated Fair Value	29,132	12,651
Government-sponsored enterprise securities (due in one to two years)
Marketable securities:
Amortized Cost	11,000	6,999
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(1)
Estimated Fair Value	$ 11,000	$ 7,000

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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