Property and equipment: Property and equipment, Table (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property and equipment
Property, Plant and Equipment, Gross	$ 1,136,470	$ 1,070,111
Accumulated depreciation and amortization	(774,829)	(709,975)
Property, Plant and Equipment, Net	361,641	360,136
Owned assets:
Property and equipment
Property, Plant and Equipment, Gross	786,550	732,485
Accumulated depreciation and amortization	(621,162)	(571,429)
Property, Plant and Equipment, Net	165,388	161,056
Equipment
Property and equipment
Property, Plant and Equipment, Gross	476,786	445,558
Leasehold improvements
Property and equipment
Property, Plant and Equipment, Gross	182,734	169,245
System infrastructure
Property and equipment
Property, Plant and Equipment, Gross	102,502	94,110
Software
Property and equipment
Property, Plant and Equipment, Gross	9,523	9,400
Office and other equipment
Property and equipment
Property, Plant and Equipment, Gross	13,735	12,855
Owned buildings
Property and equipment
Property, Plant and Equipment, Gross	1,171	1,214
Land
Property and equipment
Property, Plant and Equipment, Gross	99	103
Indefeasible rights of use (IRUs)
Property and equipment
Property, Plant and Equipment, Gross	349,920	337,626
Accumulated depreciation and amortization	(153,667)	(138,546)
Property, Plant and Equipment, Net	$ 196,253	$ 199,080