INCOME TAXES - Reconciliation of income tax expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Tax benefit at statutory rate	$ (23,591)	$ (20,608)
Other	(294)	128
Stock based compensation	2,674	2,213
Issuance costs on warrants, private placement option, and preferred stock	4,657	0
Deferred tax valuation allowances	19,542	21,606
Research and development credit	(2,988)	(3,339)
Income tax expense	$ 0	$ 0