INCOME TAXES - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
NOL carryforwards	$ 57,935	$ 50,727
R&D credit carryforwards	3,787	4,385
Accruals and other	3,811	2,464
Capitalized start-up costs	70	91
Other	28	57
Gross deferred tax assets	65,631	57,724
Deferred tax liabilities:
IPR&D	(12,528)	(12,528)
Property and equipment	0	0
Gross deferred tax liabilities	(12,528)	(12,528)
Valuation allowance	(65,631)	(57,724)
Net Deferred Tax Liability	$ (12,528)	$ (12,528)