Property, Plant and Equipment (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 1,354,318	$ 1,281,387
Less accumulated amortization and depreciation	684,609	595,060
Property, plant and equipment, net	669,709	686,327
Land
Property, plant and equipment [Abstract]
Total property, plant and equipment	6,568	6,418
Buildings and structures
Property, plant and equipment [Abstract]
Total property, plant and equipment	207,647	195,540
Cable and wire
Property, plant and equipment [Abstract]
Total property, plant and equipment	302,592	286,999
Equipment and software
Property, plant and equipment [Abstract]
Total property, plant and equipment	763,089	730,228
Plant in service
Property, plant and equipment [Abstract]
Total property, plant and equipment	1,279,896	1,219,185
Plant under construction
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 74,422	$ 62,202
Minimum | Buildings and structures
Property, Plant and Equipment [Line Items]
Useful life (in years)	10 years
Minimum | Cable and wire
Property, Plant and Equipment [Line Items]
Useful life (in years)	4 years
Minimum | Equipment and software
Property, Plant and Equipment [Line Items]
Useful life (in years)	2 years
Maximum | Buildings and structures
Property, Plant and Equipment [Line Items]
Useful life (in years)	40 years
Maximum | Cable and wire
Property, Plant and Equipment [Line Items]
Useful life (in years)	40 years
Maximum | Equipment and software
Property, Plant and Equipment [Line Items]
Useful life (in years)	17 years