INCOME TAXES - Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 32,792	$ 24,137
Tax credits	10,812	6,571
Accruals and reserves	829	854
Stock-based expense	1,202	876
Start-up costs	605	698
Gross deferred tax assets	46,240	33,136
Valuation allowance	(46,240)	(33,136)
Net deferred tax assets	$ 0	$ 0