STOCK-BASED COMPENSATION - Schedule of non-cash compensation expense related to equity awards (Details ) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Non-cash compensation expense related to equity awards
Total non-cash compensation	$ 16,436	$ 11,917
Cost of revenue
Non-cash compensation expense related to equity awards
Total non-cash compensation	397	242
Selling and marketing expense
Non-cash compensation expense related to equity awards
Total non-cash compensation	1,802	1,156
General and administrative expense
Non-cash compensation expense related to equity awards
Total non-cash compensation	12,171	9,123
Product development
Non-cash compensation expense related to equity awards
Total non-cash compensation	$ 2,066	$ 1,396