Property, Plant and Equipment (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross		$ 3,208,816,000	$ 3,089,380,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		947,916,000	906,717,000
Total property, plant, and equipment, net		2,260,900,000	2,182,663,000
Property, Plant, Equipment non-cash		9,547,000	8,985,000
Regulatory Liability, Noncurrent		349,319,000	339,287,000
Acquired Equipment Capital Leases		0	0
Asset Removal Costs [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Regulatory Liability, Noncurrent	[1]	341,107,000	327,047,000
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross		2,843,243,000	2,745,485,000
Public Utilities, Property, Plant and Equipment, Construction Work in Progress		62,264,000	39,288,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		903,096,000	867,377,000
Total property, plant, and equipment, net		$ 2,002,411,000	$ 1,917,396,000
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		$ 299,378,000	$ 296,839,000
Public Utilities, Property, Plant and Equipment, Construction Work in Progress		3,931,000	7,768,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment		44,820,000	39,340,000
Total property, plant, and equipment, net		$ 258,489,000	$ 265,267,000
Weighted Average Depreciation Rates Non Utility		2.00%	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.