STOCK-BASED COMPENSATION - Schedule of non-cash compensation expense related to equity awards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total non-cash compensation	$ 53,733	$ 52,167	$ 44,365
Cost of revenue
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total non-cash compensation	1,319	755	378
Selling and marketing expense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total non-cash compensation	6,240	5,785	3,568
General and administrative expense
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total non-cash compensation	39,650	39,177	34,325
Product development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total non-cash compensation	$ 6,524	$ 6,450	$ 6,094