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Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Jan. 01, 2019
Dec. 31, 2018
Property, plant and equipment [Abstract]      
Total property, plant and equipment $ 1,429,728   $ 1,403,118
Less accumulated amortization and depreciation 727,748   701,759
Property, plant and equipment, net 701,980 $ 703,148 701,359
Land      
Property, plant and equipment [Abstract]      
Total property, plant and equipment 6,937   6,723
Buildings and structures      
Property, plant and equipment [Abstract]      
Total property, plant and equipment 222,052   213,657
Cable and wire      
Property, plant and equipment [Abstract]      
Total property, plant and equipment 322,403   309,928
Equipment and software      
Property, plant and equipment [Abstract]      
Total property, plant and equipment 803,661   791,401
Plant in service      
Property, plant and equipment [Abstract]      
Total property, plant and equipment 1,355,053   1,321,709
Plant under construction      
Property, plant and equipment [Abstract]      
Total property, plant and equipment $ 74,675   $ 81,409
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