SHARE-BASED COMPENSATION - Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Outstanding at beginning of period (in shares)	6,236,000	3,942,000	2,696,000
Granted (in shares)	4,044,000	3,986,000	3,546,000
Exercised (in shares)	(12,000)	(90,000)	(439,000)
Cancelled or forfeited (in shares)	(121,000)	(1,602,000)	(1,861,000)
Outstanding at end of period (in shares)	10,147,000	6,236,000	3,942,000	2,696,000
Exercisable at end of period (in shares)	4,228,000
Weighted-Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 1.52	$ 2.12	$ 3.16
Granted (in dollars per share)	0.87	1.02	1.71
Exercised (in dollars per share)	1.13	1.10	0.62
Cancelled or forfeited (in dollars per share)	1.04	1.78	3.21
Outstanding at end of period (in dollars per share)	1.26	$ 1.52	$ 2.12	$ 3.16
Exercisable at end of period (in dollars per share)	$ 1.63
Weighted-Average Remaining Contractual  Life (in years)
Remaining contractual life of stock options outstanding	8 years 6 months	8 years 9 months 29 days	9 years 1 month 20 days	8 years 6 months 18 days
Exercisable at end of period	7 years 10 months 6 days
Outstanding intrinsic value beginning of period	$ 3,160	$ 358	$ 57	$ 146
Aggregate intrinsic value exercisable	$ 931