Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 906,683	$ 868,827
Less: accumulated amortization and depreciation	438,300	428,400
Property, plant and equipment, net	468,383	440,427
Land
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 3,771	3,909
Land improvements
Property, plant and equipment [Abstract]
Estimated useful lives	10 years
Total property, plant and equipment	$ 3,141	2,910
Buildings and structures
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 92,974	91,335
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	$ 407,341	390,209
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	$ 329,087	331,047
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	$ 836,314	819,410
Plant under construction
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 70,369	$ 49,417