STOCK AWARD PLAN AND STOCK-BASED COMPENSATION - Stock Option Activity for Employees and Non-Employees (Details) - Employees and Non-Employees Stock Option - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Shares Issuable Pursuant to Stock Options
Outstanding at the beginning of period (in shares)	6,132,650
Granted (in shares)	2,658,500
Exercised (in shares)	(129,978)
Forfeited (in shares)	(163,125)
Outstanding at the end of the period (in shares)	8,498,047	6,132,650
Exercisable at the end of the period (in shares)	3,745,660
Available for future grant (in shares)	769,779
Weighted-Average Exercise Price
Outstanding at the beginning of period (in dollars per share)	$ 6.54
Granted (in dollars per share)	7.92
Exercised (in dollars per share)	5.05
Forfeited (in dollars per share)	6.97
Outstanding at the end of the period (in dollars per share)	6.99	$ 6.54
Exercisable at the end of the period (in dollars per share)	$ 6.44
Weighted-Average Remaining Contractual Term Outstanding (years)	8 years 1 month 6 days	8 years 4 months 24 days
Weighted-Average Remaining Contractual Term Exercisable (years)	6 years 8 months 12 days
Total Intrinsic Value Outstanding	$ 5,214
Total Intrinsic Value Exercisable	$ 3,344