SHARE-BASED COMPENSATION - Expense (Details) - Stock options - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Employees and directors
Share-Based Compensation Awards
Stock-based compensation expenses	$ 1,324	$ 395
Employees and directors | Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses	263	67		$ 192
Employees and directors | General and administrative expense
Share-Based Compensation Awards
Stock-based compensation expenses	$ 1,061	$ 328		694
Employees, non-employees, directors
Share-Based Compensation Awards
Stock-based compensation expenses			$ 3,124	1,471
Employees, non-employees, directors | Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses			577
Employees, non-employees, directors | General and administrative expense
Share-Based Compensation Awards
Stock-based compensation expenses			$ 2,547
Nonemployees | Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses				$ 585