Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	$ 41,577	$ 98,305
Regulatory Liability, Noncurrent		$ 233,454	229,584
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Plant lives
Regulatory Liability, Current		$ 0	0
Regulatory Liability, Noncurrent		$ 208,638	203,954
Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		One to two years
Regulatory Liability, Current		$ 5,093	20,647
Regulatory Liability, Noncurrent		$ 3,602	1,080
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Various
Regulatory Liability, Current		$ 0	2,640
Regulatory Liability, Noncurrent		$ 2,530	2,584
Deferred Electric Energy Costs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Less than one year
Regulatory Liability, Current		$ 32,451	61,041
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]	$ 1,955	9,387
Regulatory Liability, Noncurrent	[2]	$ 0	0
Renewable Resources and Environmental Initiatives
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		One to two years
Regulatory Liability, Current		$ 1,075	2,960
Regulatory Liability, Noncurrent		$ 0	880
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Various
Regulatory Liability, Current		$ 1,003	1,630
Regulatory Liability, Noncurrent		18,684	21,086
Other Current Liabilities
Regulatory Liabilities [Line Items]
Entity's Recorded Provision for Revenue Subject To Refund		$ 0	$ 3,900

[1] (b) Revenue subject to refund of $0 million and $3.9 million for 2016 and 2015, respectively, is included in other current liabilities.

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