STOCK-BASED COMPENSATION - Schedule of non-cash compensation expense related to equity awards (Details ) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Non-cash compensation expense related to equity awards
Total non-cash compensation	$ 17,074	$ 14,161	$ 51,804	$ 39,236
Cost of revenue
Non-cash compensation expense related to equity awards
Total non-cash compensation	371	372	1,231	947
Selling and marketing expense
Non-cash compensation expense related to equity awards
Total non-cash compensation	1,805	1,678	5,583	4,431
General and administrative expense
Non-cash compensation expense related to equity awards
Total non-cash compensation	13,233	10,356	38,658	29,208
Product development
Non-cash compensation expense related to equity awards
Total non-cash compensation	$ 1,665	$ 1,755	$ 6,332	$ 4,650