Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 755,103	$ 693,399
Less accumulated amortization and depreciation	345,085	287,492
Property, plant and equipment, net	410,018	405,907
Equipment exchange credit for new equipment purchase	711	863	$ 14,533
Land [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	4,181	3,700
Building and Structures [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 108,341	103,341
Building and Structures [Member] | Minimum [Member]
Property, plant and equipment [Abstract]
Estimated useful lives	10 years
Building and Structures [Member] | Maximum [Member]
Property, plant and equipment [Abstract]
Estimated useful lives	40 years
Cable and Wire [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 214,721	201,938
Cable and Wire [Member] | Minimum [Member]
Property, plant and equipment [Abstract]
Estimated useful lives	4 years
Cable and Wire [Member] | Maximum [Member]
Property, plant and equipment [Abstract]
Estimated useful lives	40 years
Equipment and Software [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 391,260	366,342
Equipment and Software [Member] | Minimum [Member]
Property, plant and equipment [Abstract]
Estimated useful lives	2 years
Equipment and Software [Member] | Maximum [Member]
Property, plant and equipment [Abstract]
Estimated useful lives	16 years 8 months 12 days
Plant in Service [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 718,503	675,321
Plant under Construction [Member]
Property, plant and equipment [Abstract]
Total property, plant and equipment	$ 36,600	$ 18,078