Acquisitions - ACE Wind (Details) $ in Millions		12 Months Ended
		Dec. 31, 2016 USD ($)	Dec. 31, 2014 USD ($) MW	Dec. 31, 2015 USD ($)
Liabilities Assumed [Abstract]
Power Sales Agreements - Non-Current Liability		$ 113.8		$ 138.1
Goodwill		131.2	$ 2.9	130.6
Power Sales Agreements - Current Liability		24.6		$ 23.3
Purchase of Non-Controlling Interest		$ 6.7	$ 6.0
Lake Benton [Member]
Business Acquisition [Line Items]
Name of Acquired Entity			Lake Benton
Generating Capacity (MW) | MW			104
Storm Lake II [Member]
Business Acquisition [Line Items]
Name of Acquired Entity			Storm Lake II
Generating Capacity (MW) | MW			77
Condon [Member]
Business Acquisition [Line Items]
Name of Acquired Entity			Condon
Generating Capacity (MW) | MW			50
ACE Wind [Member]
Business Acquisition [Line Items]
Payments to Acquire Business			$ 26.9
Number of Wind Energy Facilities Acquired			3
Assets Acquired [Abstract]
Cash and Cash Equivalents			$ 3.8
Other Current Assets			14.3
Property, Plant and Equipment			156.9
Other Non-Current Assets	[1]		7.5
Total Assets Acquired			182.5
Liabilities Assumed [Abstract]
Current Liabilities	[2]		15.2
Long-Term Debt Due Within One Year			2.2
Long-Term Debt			21.1
Power Sales Agreements - Non-Current Liability			99.4
Other Non-Current Liabilities			10.6
Non-Controlling Interest	[3]		7.1
Total Liabilities and Non-Controlling Interest Assumed			155.6
Net Identifiable Assets Acquired			26.9
Goodwill			2.9
Tax Deductible Goodwill			0.0
Power Sales Agreements - Current Liability			12.4
Acquisition-Related Costs			1.4
Purchase of Non-Controlling Interest			6.0
Gain or Loss Recognized in Net Income or Other Comprehensive Income			0.0
Armenia Mountain [Member] | ACE Wind [Member]
Liabilities Assumed [Abstract]
Purchase option			$ 0.3

[1] Included in Other Non-Current Assets was $0.3 million for the option to purchase Armenia Mountain, and goodwill of $2.9 million. For tax purposes, the purchase price allocation resulted in no allocation to goodwill.

[2] Current Liabilities included $12.4 million related to the current portion of PSAs.

[3] The purchase price accounting valued the non-controlling interest related to Lake Benton, Storm Lake II and Condon at fair value using the discounted cash flow method.