Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	$ 3,042,671	$ 2,992,560	$ 2,965,226
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(893,722)	(870,967)	(879,296)
Total property, plant, and equipment, net	2,148,949	2,121,593	2,085,930
Property, Plant, Equipment non-cash	6,081	8,757	9,826
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	2,701,010	2,661,097	2,624,774
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	38,024	24,886	36,798
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(857,373)	(836,510)	(847,828)
Total property, plant, and equipment, net	1,881,661	1,849,473	1,813,744
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, Plant, and Equipment, Gross	296,046	297,295	297,269
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	7,591	9,282	6,385
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(36,349)	(34,457)	(31,468)
Total property, plant, and equipment, net	$ 267,288	$ 272,120	$ 272,186