NET LOSS PER SHARE - Basic and diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
NET LOSS PER SHARE
Net loss (numerator)	$ (18,036)	$ (15,934)	$ (10,305)	$ (7,483)	$ (33,970)	$ (17,788)
Weighted-average shares (denominator)	34,193,666		26,749,666		34,193,666	26,749,666
Basic and diluted net loss per share (in dollars per share)	$ (0.53)		$ (0.39)		$ (0.99)	$ (0.66)