NET LOSS PER SHARE - Schedule of Computation of Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net loss	$ (22,626)	$ (17,912)	$ (14,571)	$ (12,145)	$ (9,875)	$ (9,268)	$ (12,287)	$ (3,828)	$ (67,254)	$ (35,258)
Weighted average common shares outstanding, basic and diluted									32,907,100	2,075,753
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.63)	$ (0.50)	$ (0.41)	$ (0.51)	$ (4.63)	$ (4.50)	$ (5.99)	$ (1.87)	$ (2.05)	$ (16.99)
Unvested restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Unvested restricted common stock (in shares)	54,336				0				54,336	0	132,645