Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 68,835		$ 41,577
Regulatory Liability, Noncurrent		$ 784,564		233,454
Excess deferred taxes - TCJA
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Various
Regulatory Liability, Current		$ 0	[1]	0
Regulatory Liability, Noncurrent		563,662	[1]	0
Revaluation of Non-plant ADIT		$ 28,000
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Plant lives
Regulatory Liability, Current		$ 0		0
Regulatory Liability, Noncurrent		$ 196,875		208,638
Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		One to two years
Regulatory Liability, Current		$ 6,825		5,093
Regulatory Liability, Noncurrent		$ 6,503		3,602
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Various
Regulatory Liability, Current		$ 0		0
Regulatory Liability, Noncurrent		$ 2,476		2,530
Deferred Electric Energy Costs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Less than one year
Regulatory Liability, Current		$ 48,460		32,451
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]	$ 12,723		1,955
Regulatory Liability, Noncurrent	[2]	$ 0		0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period		Various
Regulatory Liability, Current		$ 827		2,078
Regulatory Liability, Noncurrent		$ 15,048		$ 18,684

[1] (a) Primarily relates to the revaluation of recoverable/regulated plant ADIT and $28.0 million revaluation impact of non-plant ADIT at Dec. 31, 2017.

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