ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll	$ 2,104	$ 2,032
Accrued patient treatment costs	954	1,162
Accrued manufacturing costs	273	2,230
Accrued professional services	639	654
Accrued obligations under material supply agreements	1,123	1,121
Accrued other	2,015	2,571
Total accrued expenses and other current liabilities	$ 7,108	$ 9,770