STOCK-BASED COMPENSATION - Stock-based compensation expenses (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Total stock compensation expenses	$ 1,270,000	$ 1,184,000	$ 3,974,536	$ 4,073,505
Research and development
Total stock compensation expenses	636,000	553,000	1,945,000	1,844,000
General and administrative
Total stock compensation expenses	$ 634,000	$ 631,000	$ 2,030,000	$ 2,230,000