Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	$ 3,247,177	$ 3,208,816	$ 3,115,854
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	960,336	947,916	919,187
Total property, plant, and equipment, net	2,286,841	2,260,900	2,196,667
Property, Plant, Equipment non-cash	11,564	9,547	8,424
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	2,867,271	2,843,243	2,760,188
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	76,631	62,264	51,014
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	914,179	903,096	878,364
Total property, plant, and equipment, net	2,029,723	2,002,411	1,932,838
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	299,324	299,378	296,826
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	3,951	3,931	7,826
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	46,157	44,820	40,823
Total property, plant, and equipment, net	$ 257,118	$ 258,489	$ 263,829