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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Included in cash and cash equivalents:
Amortized Cost	$ 51,540
Gross Unrealized Losses	(1)
Estimated Fair Value	51,539
Restricted cash:
Amortized Cost	363	$ 363
Estimated Fair Value	363
Marketable securities:
Amortized Cost	181,476	249,659
Gross Unrealized Gains	62	111
Gross Unrealized Losses	(58)	(33)
Estimated Fair Value	181,480	249,737
Money market funds
Included in cash and cash equivalents:
Amortized Cost	33,293	4,356
Estimated Fair Value	33,293	4,356
Restricted cash:
Amortized Cost	92	92
Estimated Fair Value	92	92
Commercial paper
Included in cash and cash equivalents:
Amortized Cost	18,247
Gross Unrealized Losses	(1)
Estimated Fair Value	18,246
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	10,604	5,608
Gross Unrealized Gains	5	2
Estimated Fair Value	10,609	5,610
U.S. Treasury securities (due in one to two years)
Marketable securities:
Amortized Cost	8,562	5,093
Gross Unrealized Gains		2
Gross Unrealized Losses	(1)
Estimated Fair Value	8,561	5,095
Government-sponsored enterprise securities (due in less than one year)
Marketable securities:
Amortized Cost	11,249	5,249
Gross Unrealized Gains	4	3
Estimated Fair Value	11,253	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	(5)	(1)
Estimated Fair Value	4,995	23,505
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	64,430	112,388
Gross Unrealized Gains	15	29
Gross Unrealized Losses	(9)	(8)
Estimated Fair Value	64,436	112,409
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	54,637	63,051
Gross Unrealized Gains	38	35
Gross Unrealized Losses	(14)	(7)
Estimated Fair Value	54,661	63,079
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	26,994	34,771
Gross Unrealized Gains		33
Gross Unrealized Losses	(29)	(17)
Estimated Fair Value	$ 26,965	$ 34,787

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