INCOME TAXES - Summary of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 82,703	$ 8,918
Research and development credit	15,509	3,207
Depreciable assets	10,443	0
Accrued expenses	719	0
Deferred revenue	449	2,040
Net book value of intangible assets	415	75
Stock-based compensation	332	520
Other	63	514
Net deferred tax asset	110,633	15,274
Less: valuation allowance	(110,633)	(15,274)
Net deferred tax assets	$ 0	$ 0