INCOME TAXES - Schedule of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 82,252	$ 45,215
Research and development credits	8,968	4,090
Capitalized assets	79	23
Accruals and reserves	651	390
Operating lease liabilities	2,070	806
Stock-based compensation	4,250	0
Gross deferred tax assets	98,270	50,524
Operating lease right-of-use assets	(1,969)	0
Gross deferred tax liabilities	(1,969)	0
Total net deferred tax assets	96,301	50,524
Less: valuation allowance	(96,301)	(50,524)
Net deferred tax assets	$ 0	$ 0