FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Included in cash and cash equivalents:
Amortized Cost	$ 20,624	$ 10,661
Gross Unrealized Gains	4
Estimated Fair Value	20,628	10,661
Restricted cash:
Amortized Cost	363	270
Estimated Fair Value	363
Marketable securities:
Amortized Cost	241,919	145,200
Gross Unrealized Gains	258	157
Gross Unrealized Losses	(73)	(25)
Estimated Fair Value	242,104	145,332
Money market funds
Included in cash and cash equivalents:
Amortized Cost	12,629	6,671
Estimated Fair Value	12,629	6,671
Restricted cash:
Amortized Cost	92
Estimated Fair Value	92
Commercial paper
Included in cash and cash equivalents:
Amortized Cost	7,995	3,990
Gross Unrealized Gains	4
Estimated Fair Value	7,999	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	3,002	6,506
Gross Unrealized Gains	7	6
Estimated Fair Value	3,009	6,512
Government-sponsored enterprise securities (due in one to two years)
Marketable securities:
Amortized Cost	6,501	6,999
Gross Unrealized Gains		1
Gross Unrealized Losses	(1)
Estimated Fair Value	6,500	7,000
Commercial paper (due in less than one year)
Marketable securities:
Amortized Cost	97,225	40,110
Gross Unrealized Gains	49	33
Gross Unrealized Losses	(31)	(3)
Estimated Fair Value	97,243	40,140
Corporate notes (due in less than one year)
Marketable securities:
Amortized Cost	95,449	78,926
Gross Unrealized Gains	198	116
Gross Unrealized Losses	(18)	(13)
Estimated Fair Value	95,629	79,029
Corporate notes (due in one to two years)
Marketable securities:
Amortized Cost	39,742	12,659
Gross Unrealized Gains	4	1
Gross Unrealized Losses	(23)	(9)
Estimated Fair Value	$ 39,723	$ 12,651

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