Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	$ 98,305	$ 87,723
Regulatory Liability, Noncurrent		$ 229,584	115,188
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Plant lives
Regulatory Liability, Current		$ 0	0
Regulatory Liability, Noncurrent		$ 203,954	68,106
Revenue Subject to Refund [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		One to two years
Regulatory Liability, Current		$ 20,647	7,527
Regulatory Liability, Noncurrent		$ 1,080	19,518
Renewable Resources and Environmental Initiatives
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		One to two years
Regulatory Liability, Current		$ 2,960	1,120
Regulatory Liability, Noncurrent		$ 880	765
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Various
Regulatory Liability, Current		$ 2,640	2,577
Regulatory Liability, Noncurrent		$ 2,584	4,468
Deferred Electric Energy Costs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Less than one year
Regulatory Liability, Current		$ 61,041	53,971
Regulatory Liability, Noncurrent		$ 0	0
Contract Valuation Adjustments
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Term of related contract
Regulatory Liability, Current	[2]	$ 9,387	20,211
Regulatory Liability, Noncurrent	[2]	$ 0	2,521
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Less than one year
Regulatory Liability, Current	[3]	$ 645	1,425
Regulatory Liability, Noncurrent	[3]	$ 0	0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Various
Regulatory Liability, Current		$ 985	892
Regulatory Liability, Noncurrent		21,086	19,810
Other Current Liabilities [Member]
Regulatory Liabilities [Line Items]
Entity's Recorded Provision for Revenue Subject To Refund		$ 3,900	$ 51,200

[1] Revenue subject to refund of $3.9 million and $51.2 million for 2015 and 2014, respectively, is included in other current liabilities.

[2] Includes the fair value of certain long-term PPAs used to meet energy capacity requirements.

[3] Includes costs for conservation programs as well as incentives allowed in certain jurisdictions.