INCOME TAXES - Significant components of deferred taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets (liabilities):
Federal net operating loss carryforward	$ 81,960	$ 63,624
Stock compensation	3,270	4,533
Intangible assets	8,077	8,392
Research and development credit	16,601	13,612
Operating lease right-of-use assets	(1,229)
Lease liabilities	1,538
Other	2,336	2,858
Total deferred tax assets, net of deferred tax liabilities	112,553	93,019
Valuation allowance	(112,553)	(93,019)
Net deferred tax assets	$ 0	$ 0