NET LOSS PER SHARE - Schedule of basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Earnings Per Share [Abstract]
Net loss	$ (23,849)	$ (18,535)	$ (19,622)	$ (15,454)	$ (14,256)	$ (14,261)	$ (62,006)	$ (43,971)
Less net income attributable to non-controlling interests	21			25			63	75
Net loss attributable to shareholders	$ (23,870)			$ (15,479)			$ (62,069)	$ (44,046)
Weighted average shares outstanding, basic and diluted (in shares)	23,671			20,206			23,703	19,968
Net loss per share, basic and diluted (in dollars per share)	$ (1.01)			$ (0.77)			$ (2.62)	$ (2.21)
Anti-dilutive securities excluded from diluted net loss per share calculation for stock-based compensation plans (in shares)	1,920			1,065			1,920	1,065