STOCK BASED COMPENSATION - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Stock option activity
Outstanding	2,326,801
Granted	1,261,373
Exercised	(131,755)
Cancelled	(399,465)
Outstanding	3,056,954	2,326,801
Exercisable at end of period	940,101
Vested and expected to vest at end of period	2,909,114
Weighted average exercise price per share
Outstanding	$ 13.07
Granted	12.06
Exercised	3.25
Cancelled	13.62
Outstanding	13.00	$ 13.07
Exercisable at end of period	12.80
Vested and expected to vest at end of period	$ 13.08
Stock option activity, additional information
Outstanding Weighted-Average Remaining Contractual Term	8 years 7 months 6 days	8 years 8 months 12 days
Outstanding Aggregate Intrinsic Value	$ 3,604	$ 7,927
Exercisable at end of period, Weighted-Average Remaining Contractual Term	7 years 9 months 18 days
Exercisable at end of period, Aggregate Intrinsic Value	$ 1,727
Vested and expected to vest at end of period, Weighted-Average Remaining Contractual Term	8 years 6 months
Vested and expected to vest at end of period, Aggregate Intrinsic Value	$ 3,571