ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll	$ 2,032	$ 3,430
Accrued patient treatment costs	1,162	2,053
Accrued manufacturing costs	2,230	546
Accrued professional services	654	235
Accrued obligations under material supply agreements	1,121	0
Accrued construction costs	0	457
Accrued other	2,571	1,868
Total accrued expenses and other current liabilities	$ 9,770	$ 8,589