Acquisitions - ACE Wind (Details) $ in Millions		1 Months Ended		12 Months Ended
		Feb. 28, 2014 USD ($)	Jan. 31, 2014 USD ($) MW	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)
Liabilities Assumed [Abstract]
Power Purchase Agreements - Non-Current Liability	[1]			$ 110.7	$ 138.1
Goodwill				2.9	130.6
Power Purchase Agreements - Current Liability	[2]			19.4	$ 23.3
Armenia Mountain [Member]
Liabilities Assumed [Abstract]
Purchase Option			$ 0.3
ACE Wind [Member]
Business Acquisition [Line Items]
Date of Acquisition			January 2014
Payments to Acquire Businesses			$ 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	[3]		7.5
Total Assets Acquired			182.5
Liabilities Assumed [Abstract]
Current Liabilities	[4]		15.2
Long-Term Debt Due Within One Year			2.2
Long-Term Debt			21.1
Power Purchase Agreements - Non-Current Liability			99.4
Other Non-Current Liabilities			10.6
Non-Controlling Interest	[5]		7.1
Total Liabilities and Non-Controlling Interest Assumed			155.6
Net Identifiable Assets Acquired			26.9
Goodwill			2.9
Deductible Goodwill			0.0
Power Purchase Agreements - Current Liability			$ 12.4
Acquisition-Related Costs				$ 1.4
Purchase of Non-Controlling Interest		$ 6.0
Storm Lake II [Member]
Business Acquisition [Line Items]
Name of Acquired Entity			Storm Lake II
Generating Capacity (MW) | MW			77
Lake Benton [Member]
Business Acquisition [Line Items]
Name of Acquired Entity			Lake Benton
Generating Capacity (MW) | MW			104
Condon [Member]
Business Acquisition [Line Items]
Name of Acquired Entity			Condon
Generating Capacity (MW) | MW			50

[1] Power Purchase Agreements acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions. (See Note 7. Acquisitions.)

[2] Power Purchase Agreements acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions. (See Note 7. Acquisitions.)

[3] 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.

[4] Current Liabilities included $12.4 million related to the current portion of Power Purchase Agreements.

[5] 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.