LEASES - Lease Cost (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Lessee, Lease, Description [Line Items]
Fixed lease cost	$ 4,777	$ 13,884
Variable lease cost	588	1,644
Net lease cost	5,365	15,528
Short-term lease cost	100	700
Sublease income	500	1,000
Cost of revenue
Lessee, Lease, Description [Line Items]
Fixed lease cost	80	127
Selling and marketing expense
Lessee, Lease, Description [Line Items]
Fixed lease cost	2,374	6,804
Variable lease cost	337	1,007
General and administrative expense
Lessee, Lease, Description [Line Items]
Fixed lease cost	1,919	5,967
Variable lease cost	197	528
Product development expense
Lessee, Lease, Description [Line Items]
Fixed lease cost	404	986
Variable lease cost	$ 54	$ 109