ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll	$ 2,237	$ 2,032
Accrued patient treatment costs	951	1,162
Accrued manufacturing costs	408	2,230
Accrued professional services	584	654
Accrued construction costs	694	0
Accrued obligations under material supply agreements	0	1,121
Accrued other	1,803	2,571
Total accrued expenses and other current liabilities	$ 6,677	$ 9,770