Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 1,403,118	$ 1,281,387
Less accumulated amortization and depreciation	701,759	595,060
Property, plant and equipment, net	701,359	686,327
Depreciation	142,111	151,063	$ 123,995
Capital lease agreements	5,600	5,900
Unamortized capitalized software costs	27,800	28,000
Land
Property, plant and equipment [Abstract]
Total property, plant and equipment	6,723	6,418
Buildings and structures
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 213,657	195,540
Buildings and structures | Minimum
Property, plant and equipment [Abstract]
Estimated useful lives	10 years
Buildings and structures | Maximum
Property, plant and equipment [Abstract]
Estimated useful lives	40 years
Cable and wire
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 309,928	286,999
Cable and wire | Minimum
Property, plant and equipment [Abstract]
Estimated useful lives	4 years
Cable and wire | Maximum
Property, plant and equipment [Abstract]
Estimated useful lives	40 years
Equipment and software
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 791,401	730,228
Equipment and software | Minimum
Property, plant and equipment [Abstract]
Estimated useful lives	2 years
Equipment and software | Maximum
Property, plant and equipment [Abstract]
Estimated useful lives	17 years
Plant in service
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 1,321,709	1,219,185
Plant under construction
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 81,409	$ 62,202