SHARE-BASED COMPENSATION - Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Stock options | Employees and directors
Share-Based Compensation Awards
Stock-based compensation expenses	$ 2,188	$ 848
Stock options | Employees and directors | Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses	489	145
Stock options | Employees and directors | General and administrative expense
Share-Based Compensation Awards
Stock-based compensation expenses	1,699	703
RSUs
Share-Based Compensation Awards
Stock-based compensation expenses	23	16
RSUs | Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses	21	14
RSUs | General and administrative expense
Share-Based Compensation Awards
Stock-based compensation expenses	$ 2	$ 2