Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, plant and equipment
Property, plant and equipment cost	$ 1,131,271	$ 1,036,137
Less accumulated depreciation and amortization	(522,973)	(440,140)
Total property, plant and equipment, net	608,298	595,997
Depreciation expense	86,700	87,800	$ 85,400
Land
Property, plant and equipment
Property, plant and equipment cost	67,278	67,278
Buildings
Property, plant and equipment
Property, plant and equipment cost	120,999	115,775
Central office equipment
Property, plant and equipment
Property, plant and equipment cost	222,248	208,004
Outside communications plant
Property, plant and equipment
Property, plant and equipment cost	479,655	412,244
Other equipment, vehicles and furniture
Property, plant and equipment
Property, plant and equipment cost	104,846	87,849
Construction in progress
Property, plant and equipment
Property, plant and equipment cost	34,759	55,606
Software
Property, plant and equipment
Property, plant and equipment cost	90,652	78,547
Other property
Property, plant and equipment
Property, plant and equipment cost	$ 10,834	$ 10,834