NET LOSS PER SHARE - Basic and diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
NET LOSS PER SHARE
Net loss (numerator)	$ (40,393)	$ (26,179)	$ (18,036)	$ (15,934)	$ (66,572)	$ (33,970)
Weighted-average shares (denominator)	39,972,174		34,193,666		39,862,878	34,193,666
Basic and diluted net loss per share (in dollars per share)	$ (1.01)		$ (0.53)		$ (1.67)	$ (0.99)