Property, Plant and Equipment (Details) - USD ($)		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross		$ 3,089,380,000	$ 2,992,560,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		906,717,000	870,967,000
Total property, plant, and equipment, net		2,182,663,000	2,121,593,000
Property, Plant, Equipment non-cash		8,985,000	8,757,000
Regulatory Liability, Noncurrent		339,287,000	317,205,000
Acquired Equipment Capital Leases		0	1,300,000
Asset Removal Costs [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Regulatory Liability, Noncurrent	[1]	327,047,000	311,238,000
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross		2,745,485,000	2,661,097,000
Public Utilities, Property, Plant and Equipment, Construction Work in Progress		39,288,000	24,886,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		867,377,000	836,510,000
Total property, plant, and equipment, net		$ 1,917,396,000	$ 1,849,473,000
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service		2.80%	2.80%	2.80%
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross		$ 296,839,000	$ 297,295,000
Public Utilities, Property, Plant and Equipment, Construction Work in Progress		7,768,000	9,282,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		39,340,000	34,457,000
Total property, plant, and equipment, net		$ 265,267,000	$ 272,120,000
Weighted Average Depreciation Rates Non Utility		2.20%	2.20%	2.20%

[1] Estimated costs of removal on certain regulated properties are collected through rates. See "Accounting Policies—Plant, Property, and Accrued Asset Removal Costs" below.