Regulatory Matters (Details) $ in Thousands	1 Months Ended		3 Months Ended
	Mar. 31, 2016 USD ($)	May 31, 2014 MW	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Regulatory Matters [Line Items]
Approved increase to basic rates and charges	$ 30,800
Wholesale sales margins, percent shared with customers	50.00%
Total current regulatory assets			$ 32,040	$ 33,912
Total long-term regulatory assets			444,209	450,710
Total current regulatory liabilities			4,829	7,704
Total long-term regulatory liabilities			$ 676,676	$ 670,294
Benchmark for Wholesale Sales Margin	$ 6,300
Capacity Sales, Percent Shared with Customers	50.00%
Deferred fuel [Member]
Regulatory Matters [Line Items]
Regulatory liability recovery period			1 year
FTRs [Member]
Regulatory Matters [Line Items]
Regulatory liability recovery period			1 year
DSM program costs [Member]
Regulatory Matters [Line Items]
Regulatory liability recovery period			1 year
Environmental projects [Member]
Regulatory Matters [Line Items]
Regulatory asset recovery period			1 year
Deferred MISO Non-fuel Costs [Member]
Regulatory Matters [Line Items]
Total current regulatory assets			$ 11,767
Regulatory asset recovery period			1 year
Other miscellaneous [Member]
Regulatory Matters [Line Items]
Regulatory asset recovery period			1 year
Deferred MISO Costs [Member]
Regulatory Matters [Line Items]
Total regulatory assets			$ 117,700
LT - Other Miscellaneous [Member]
Regulatory Matters [Line Items]
Regulatory asset recovery period			2 years
Eagle Valley [Member]
Regulatory Matters [Line Items]
Public Utilities, Approved Production Plant Capacity, Minimum | MW		644
Public Utilities, Approved Production Plan Capacity , Maximum | MW		685
Increase to Annual Depreciation Rate [Member]
Regulatory Matters [Line Items]
Approved increase to basic rates and charges	$ 24,300