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	$ 20,712		$ 17,428
Regulatory Liability, Noncurrent	$ 386,807		148,189
Excess deferred taxes - TCJA
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory Liability, Current	$ 0	[1]	0
Regulatory Liability, Noncurrent	236,589	[1]	0
Revaluation of Non-plant ADIT	$ 41,000
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Plant lives
Regulatory Liability, Current	$ 0		0
Regulatory Liability, Noncurrent	$ 146,370		139,735
Deferred Electric Production And Natural Gas Costs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory Liability, Current	$ 13,950		11,377
Regulatory Liability, Noncurrent	$ 0		0
DOE Settlement
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory Liability, Current	$ 5,261		4,822
Regulatory Liability, Noncurrent	$ 0		0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory Liability, Current	$ 1,501		1,229
Regulatory Liability, Noncurrent	$ 3,848		$ 8,454

[1] (a) Primarily relates to the revaluation of recoverable/regulated plant ADIT and $41.0 million revaluation impact of non-plant ADIT at Dec. 31, 2017.