SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (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
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Total operating expenses	23,183	18,480	15,028	12,879	10,344	8,148	6,999	3,841	69,570	29,332
Loss from operations	(23,183)	(18,480)	(15,028)	(12,879)	(10,344)	(8,148)	(6,999)	(3,841)	(69,570)	(29,332)
Net loss	$ (22,626)	$ (17,912)	$ (14,571)	$ (12,145)	$ (9,875)	$ (9,268)	$ (12,287)	$ (3,828)	$ (67,254)	$ (35,258)
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)