FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Included in cash and cash equivalents:
Amortized Cost	$ 24,805
Estimated Fair Value	24,805
Restricted cash:
Amortized Cost	363	$ 363
Estimated Fair Value	363	363
Marketable securities:
Amortized Cost	186,430	249,659
Gross Unrealized Gains	54	111
Gross Unrealized Losses	(38)	(33)
Estimated Fair Value	186,446	249,737
Money market funds
Included in cash and cash equivalents:
Amortized Cost	19,805	4,356
Estimated Fair Value	19,805	4,356
Restricted cash:
Amortized Cost	92	92
Estimated Fair Value	92	92
Commercial paper
Included in cash and cash equivalents:
Amortized Cost	5,000
Estimated Fair Value	5,000
Certificate of deposit
Restricted cash:
Amortized Cost	271	271
Estimated Fair Value	271	271
U.S. Treasury securities (due in less than one year)
Marketable securities:
Amortized Cost	14,056	5,608
Gross Unrealized Gains	3	2
Estimated Fair Value	14,059	5,610
U.S. Treasury securities (due in one to two years)
Marketable securities:
Amortized Cost	7,099	5,093
Gross Unrealized Gains	2	2
Estimated Fair Value	7,101	5,095
Municipal securities (due in one to two years)
Marketable securities:
Amortized Cost	3,000
Gross Unrealized Losses	(1)
Estimated Fair Value	2,999
Government-sponsored enterprise securities (due in less than one year)
Marketable securities:
Amortized Cost	11,250	5,249
Gross Unrealized Gains	2	3
Estimated Fair Value	11,252	5,252
Government-sponsored enterprise securities (due in one to two years)
Marketable securities:
Amortized Cost	5,000	23,499
Gross Unrealized Gains		7
Gross Unrealized Losses	(3)	(1)
Estimated Fair Value	4,997	23,505
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	72,440	112,388
Gross Unrealized Gains	10	29
Gross Unrealized Losses	(10)	(8)
Estimated Fair Value	72,440	112,409
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	49,309	63,051
Gross Unrealized Gains	32	35
Gross Unrealized Losses	(8)	(7)
Estimated Fair Value	49,333	63,079
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	24,276	34,771
Gross Unrealized Gains	5	33
Gross Unrealized Losses	(16)	(17)
Estimated Fair Value	$ 24,265	$ 34,787

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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