STOCKHOLDERS' EQUITY - Share-based compensation (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
STOCKHOLDERS' EQUITY
Stock-based compensation expense	$ 434,367	$ 752,467	$ 926,698	$ 2,244,963
Unrecognized compensation expense related to unvested Employee Awards	$ 404,262		$ 404,262
Recognition period related to unvested Employee Awards			11 months
Number of Shares
Outstanding, beginning of period			10,883,780
Granted			1,377,020
Forfeited or expired			(1,820,548)
Outstanding, end of period	10,440,252		10,440,252
Exercisable, end of period	7,072,932		7,072,932
Weighted-average Exercise Price per Share
Outstanding, beginning of period			$ 1.28
Granted			0.54
Forfeited or expired			1.68
Outstanding, end of period	$ 1.11		1.11
Exercisable, end of period	$ 1.41		$ 1.41
Weighted-average Remaining Contractual Term (in years)
Outstanding, end of period			5 years 2 months 12 days
Exercisable, end of period			5 years 7 months 6 days