Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 868,827	$ 736,491
Less: accumulated amortization and depreciation	428,400	373,404
Property, plant and equipment, net	440,427	363,087
Land
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 3,909	3,677
Land improvements
Property, plant and equipment [Abstract]
Estimated useful lives	10 years
Total property, plant and equipment	$ 2,910	2,620
Buildings and structures
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 91,335	85,835
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	$ 390,209	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	30 years
Equipment and software
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 331,047	278,873
Equipment and software | Minimum
Property, plant and equipment [Abstract]
Estimated useful lives	4 years
Equipment and software | Maximum
Property, plant and equipment [Abstract]
Estimated useful lives	8 years
Plant in service
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 819,410	705,265
Plant under construction
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 49,417	$ 31,226