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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Dec. 31, 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	249,659	145,200
Gross Unrealized Gains	111	157
Gross Unrealized Losses	(33)	(25)
Estimated Fair Value	249,737	145,332
Money market funds
Included in cash and cash equivalents:
Amortized Cost	4,356	6,671
Estimated Fair Value	4,356	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
Government-sponsored enterprise securities (due in less than one year)
Marketable securities:
Amortized Cost	5,249	6,506
Gross Unrealized Gains	3	6
Estimated Fair Value	5,252	6,512
Government-sponsored enterprise securities (due in one to two years)
Marketable securities:
Amortized Cost	23,499	6,999
Gross Unrealized Gains	7	1
Gross Unrealized Losses	(1)
Estimated Fair Value	23,505	7,000
U.S. Treasury securities (due in less than one year)
Marketable securities:
Amortized Cost	5,608
Gross Unrealized Gains	2
Estimated Fair Value	5,610
U.S. Treasury securities (due in one to two years)
Marketable securities:
Amortized Cost	5,093
Gross Unrealized Gains	2
Estimated Fair Value	5,095
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	112,388	40,110
Gross Unrealized Gains	29	33
Gross Unrealized Losses	(8)	(3)
Estimated Fair Value	112,409	40,140
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	63,051	78,926
Gross Unrealized Gains	35	116
Gross Unrealized Losses	(7)	(13)
Estimated Fair Value	63,079	79,029
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	34,771	12,659
Gross Unrealized Gains	33	1
Gross Unrealized Losses	(17)	(9)
Estimated Fair Value	$ 34,787	$ 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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