STOCK-BASED COMPENSATION - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 6,764	$ 4,867	$ 12,384	$ 9,390
Research and development
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	3,578	2,763	6,417	5,061
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 3,186	$ 2,104	$ 5,967	$ 4,329