Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	$ 3,146,631	$ 3,089,380	$ 3,042,671
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	932,179	906,717	893,722
Total property, plant, and equipment, net	2,214,452	2,182,663	2,148,949
Property, Plant, Equipment non-cash	11,345	8,985	6,081
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	2,783,883	2,745,485	2,701,010
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	57,068	39,288	38,024
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	890,028	867,377	857,373
Total property, plant, and equipment, net	1,950,923	1,917,396	1,881,661
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	297,809	296,839	296,046
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	7,871	7,768	7,591
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	42,151	39,340	36,349
Total property, plant, and equipment, net	$ 263,529	$ 265,267	$ 267,288