STOCK-BASED COMPENSATION - Schedule of Stock-based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 25,168	$ 5,152	$ 876
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	24,271	4,806	876
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	447	202	0
Employee Stock Purchase Plan (ESPP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	450	144	0
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	13,547	2,643	379
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 11,621	$ 2,509	$ 497