PROPERTY AND EQUIPMENT - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 971,685	$ 772,129
Less: accumulated depreciation and amortization ($50,253 and $40,218 related to capital leases at December 31, 2016 and 2015, respectively)	(513,120)	(469,449)
Accumulated depreciation and amortization related to capital leases	50,253	40,218
Property and equipment, net	458,565	302,680
Network equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	247,190	231,579
Network equipment under capital lease
Property, Plant and Equipment [Line Items]
Property and equipment, gross	14,206	14,231
Furniture and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	26,246	18,300
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	43,930	48,011
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	412,631	396,891
Buildings under capital lease
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 227,482	$ 63,117