STOCK-BASED COMPENSATION - Employee Stock Purchase Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Stock Purchase Plan
Stock-based compensation expense	$ 30,532	$ 20,230	$ 16,162
Employee Stock Purchase Plan
Employee Stock Purchase Plan
Shares purchased (in shares)	80,789	92,738	90,084
Shares issued (in shares)		92,738	90,084
Weighted average discounted price per share (in dollars per share)	$ 18.64	$ 12.64	$ 9.74
Stock-based compensation expense	$ 390	$ 234	$ 141