NET LOSS PER SHARE - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net loss per common share:
Net loss	$ (8,748)	$ (8,465)	$ (9,059)	$ (10,873)	$ (12,467)	$ (7,910)	$ (11,098)	$ (10,185)	$ (37,145)	$ (41,660)	$ (39,216)
Basic and diluted loss per common share:
Basic and diluted (in shares)									42,739,525	41,729,013	40,237,848
Basic and diluted (in dollars per share)	$ (0.20)	$ (0.20)	$ (0.21)	$ (0.26)	$ (0.30)	$ (0.19)	$ (0.27)	$ (0.25)	$ (0.87)	$ (1.00)	$ (0.97)