NET LOSS PER SHARE - Basic and diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
NET LOSS PER SHARE
Net loss (numerator)	$ (23,922)	$ (18,036)	$ (15,934)	$ (11,426)	$ (10,305)	$ (7,483)	$ (57,892)	$ (29,214)
Weighted-average shares (denominator)	34,371,927			27,330,579			34,253,739	26,945,432
Basic and diluted net loss per share (in dollars per share)	$ (0.70)			$ (0.42)			$ (1.69)	$ (1.08)