INCOME TAXES - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
NOL carryforwards	$ 50,727	$ 43,212
R&D credit carryforwards	4,385	3,876
Accruals and other	2,464	2,011
Capitalized start-up costs	91	122
Other	57	32
Gross deferred tax assets	57,724	49,253
Deferred tax liabilities:
IPR&D	(12,528)	(12,528)
Property and equipment	0	(60)
Gross deferred tax liabilities	(12,528)	(12,588)
Valuation allowance	(57,724)	(49,193)
Net Deferred Tax Liability	$ (12,528)	$ (12,528)