NET LOSS PER SHARE - Calculation of Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net loss	$ (12,437)	$ (13,892)	$ (15,009)	$ (17,304)	$ (14,447)	$ (11,573)	$ (41,338)	$ (43,324)
Accretion of redeemable convertible preferred stock	0			0			0	(46)
Net loss attributable to common stockholders	$ (12,437)			$ (17,304)			$ (41,338)	$ (43,370)
Denominator:
Weighted average common shares outstanding, basic and diluted (in shares)	36,942,475			23,083,367			29,804,034	14,430,397
Net loss per basic and diluted (USD per share)	$ (0.34)			$ (0.75)			$ (1.39)	$ (3.01)