Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment, Gross	$ 3,072,998	$ 2,992,560	$ 2,990,662
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(905,137)	(870,967)	(883,568)
Total property, plant, and equipment, net	2,167,861	2,121,593	2,107,094
Property, Plant, Equipment non-cash	9,700	8,757	11,834
Utility Plant [Member]
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment, Gross	2,710,658	2,661,097	2,655,136
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	58,280	24,886	31,778
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(867,281)	(836,510)	(850,590)
Total property, plant, and equipment, net	1,901,657	1,849,473	1,836,324
Non Utility Plant [Member]
Property, Plant and Equipment [Line Items]
Property, Plant, and Equipment, Gross	296,169	297,295	297,199
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	7,891	9,282	6,549
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(37,856)	(34,457)	(32,978)
Total property, plant, and equipment, net	$ 266,204	$ 272,120	$ 270,770