STOCK-BASED COMPENSATION - Schedule of non-cash compensation expense related to equity awards (Details ) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Non-cash compensation expense related to equity awards
Total non-cash compensation	$ 18,294	$ 13,158	$ 34,730	$ 25,075
Cost of revenue
Non-cash compensation expense related to equity awards
Total non-cash compensation	463	333	860	575
Selling and marketing expense
Non-cash compensation expense related to equity awards
Total non-cash compensation	1,976	1,597	3,778	2,753
General and administrative expense
Non-cash compensation expense related to equity awards
Total non-cash compensation	13,254	9,729	25,425	18,852
Product development
Non-cash compensation expense related to equity awards
Total non-cash compensation	$ 2,601	$ 1,499	$ 4,667	$ 2,895