PROPERTY AND EQUIPMENT - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 759,544				$ 1,069,831
Less: accumulated depreciation and amortization ($55,198 related to capital leases at December 31, 2018)	(544,433)				(585,525)
Property and equipment, net	215,111	$ 220,645	$ 228,210	$ 233,391	484,306	$ 493,315	$ 468,428	$ 476,439	$ 460,634	$ 442,704	$ 428,741	$ 292,272
Accumulated depreciation and amortization related to capital leases					55,198
Network equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	309,323				274,680
Network equipment under capital lease
Property, Plant and Equipment [Line Items]
Property and equipment, gross	0				14,206
Furniture and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	27,681				28,583
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	50,048				66,924
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	372,492				414,212
Buildings under capital lease
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 0				$ 271,226