Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 1,523,715	$ 1,498,691
Less: accumulated amortization and depreciation	827,795	797,177
Property, plant and equipment, net	695,920	701,514
Land
Property, plant and equipment [Abstract]
Total property, plant and equipment	7,143	6,976
Buildings and structures
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 238,728	232,730
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 fiber
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 348,658	334,260
Cable and fiber | Minimum
Property, plant and equipment [Abstract]
Estimated useful lives	15 years
Cable and fiber | Maximum
Property, plant and equipment [Abstract]
Estimated useful lives	40 years
Equipment and software
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 870,089	867,898
Equipment and software | Minimum
Property, plant and equipment [Abstract]
Estimated useful lives	3 years
Equipment and software | Maximum
Property, plant and equipment [Abstract]
Estimated useful lives	20 years
Plant in service
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 1,464,618	1,441,864
Plant under construction
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 59,097	$ 56,827