STOCKHOLDERS' EQUITY, STOCK-BASED COMPENSATION AND NET LOSS PER SHARE - Summary of Activity Under Equity Incentive Plans Related to Equity-Classified Stock Option Grants (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares shares
Stock Options
Shares
Outstanding at beginning of period (in shares) | shares	10,874,849
Granted (in shares) | shares	805,250
Exercised (in shares) | shares	(2,250)
Forfeited, cancelled or expired (in shares) | shares	(1,318,770)
Outstanding at end of period (in shares) | shares	10,359,079
Options exercisable at end of period (in shares) | shares	9,748,892
Weighted-Average Exercise Price (in dollars per share)
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 4.36
Granted (in dollars per share) | $ / shares	0.73
Exercised (in dollars per share) | $ / shares	1.99
Forfeited, cancelled or expired (in dollars per share) | $ / shares	3.37
Outstanding at end of period (in dollars per share) | $ / shares	4.20
Options exercisable at end of period (in dollars per share) | $ / shares	$ 4.42
Weighted-Average Remaining Contractual Term (in years)
Outstanding at end of period	6 years 9 months 18 days
Options exercisable at end of period	6 years 7 months 6 days
Aggregate Intrinsic Value (in thousands)
Outstanding at end of period | $	$ 0
Options exercisable at end of period | $	$ 0
China
Shares
Outstanding at beginning of period (in shares) | shares	1,464,750
Granted (in shares) | shares	152,000
Forfeited, cancelled or expired (in shares) | shares	(518,000)
Outstanding at end of period (in shares) | shares	1,098,750
Options exercisable at end of period (in shares) | shares	911,500
Weighted-Average Exercise Price (in dollars per share)
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 5.60
Granted (in dollars per share) | $ / shares	1.56
Forfeited, cancelled or expired (in dollars per share) | $ / shares	5.11
Outstanding at end of period (in dollars per share) | $ / shares	5.21
Options exercisable at end of period (in dollars per share) | $ / shares	$ 5.20
Weighted-Average Remaining Contractual Term (in years)
Outstanding at end of period	6 years 4 months 24 days
Options exercisable at end of period	6 years
Aggregate Intrinsic Value (in thousands)
Outstanding at end of period | $	$ 0
Options exercisable at end of period | $	$ 0