LEASES - Lease Cost (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019
Lessee, Lease, Description [Line Items]
Fixed lease cost	$ 13,815	$ 24,307
Variable lease cost	2,402	4,721
Net lease cost	16,217	29,028
Short-term lease cost	1,700	3,000
Sublease income	500	1,100
Cost of revenue
Lessee, Lease, Description [Line Items]
Fixed lease cost	1,131	2,206
Variable lease cost	107	264
Selling and marketing expense
Lessee, Lease, Description [Line Items]
Fixed lease cost	3,002	4,911
Variable lease cost	366	670
General and administrative expense
Lessee, Lease, Description [Line Items]
Fixed lease cost	9,400	16,590
Variable lease cost	1,884	3,688
Product development expense
Lessee, Lease, Description [Line Items]
Fixed lease cost	282	600
Variable lease cost	$ 45	$ 99