Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	$ 3,177,196	$ 3,089,380	$ 3,072,998
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	943,334	906,717	905,137
Total property, plant, and equipment, net	2,233,862	2,182,663	2,167,861
Property, Plant, Equipment non-cash	8,918	8,985	9,700
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	2,815,340	2,745,485	2,710,658
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	58,470	39,288	58,280
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	899,851	867,377	867,281
Total property, plant, and equipment, net	1,973,959	1,917,396	1,901,657
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	298,586	296,839	296,169
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	4,800	7,768	7,891
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	43,483	39,340	37,856
Total property, plant, and equipment, net	$ 259,903	$ 265,267	$ 266,204