STOCKHOLDERS' EQUITY AND SHARE-BASED COMPENSATION PLANS - Expense by Classification (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense recognized	$ 1,564	$ 1,996	$ 2,887	$ 4,132
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense recognized	661	959	1,209	2,024
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense recognized	$ 903	$ 1,037	$ 1,678	$ 2,108