STOCK-BASED COMPENSATION - Summary of Stock-Based Compensation Associated with Each Type of Award (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]
Total stock-based compensation expense	$ 20,176	$ 11,432	$ 6,727
Stock options and RSUs and PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	19,216	10,691	5,559
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 960	$ 741	$ 1,168