NET LOSS PER SHARE - Basic and diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
NET LOSS PER SHARE
Net loss (numerator)	$ (23,136)	$ (23,922)	$ (18,036)	$ (15,934)	$ (14,060)	$ (11,426)	$ (10,305)	$ (7,483)	$ (81,028)	$ (43,274)
Weighted-average shares (denominator)									35,183,488	28,772,384
Basic and diluted net loss per share (in dollars per share)	$ (0.60)	$ (0.70)	$ (0.53)	$ (0.47)	$ (0.42)	$ (0.42)	$ (0.38)	$ (0.28)	$ (2.30)	$ (1.50)