STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS - Expense by Classification (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense recognized	$ 1,606	$ 1,634	$ 4,493	$ 5,762
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense recognized	668	761	1,877	2,784
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense recognized	$ 938	$ 873	$ 2,616	$ 2,978