Acquisitions - ACE Wind Acquisition (Details) $ in Millions		Feb. 11, 2014 USD ($)	Jan. 30, 2014 USD ($) MW	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Liabilities [Abstract]
Power Purchase Agreements, Non-Current	[1]			$ 149.5	$ 110.7
Power Purchase Agreements, Current	[2]			$ 24.3	$ 19.4
Armenia Mountain [Member]
Liabilities [Abstract]
Purchase option			$ 0.3
ACE Wind [Member]
Business Acquisition [Line Items]
Date of Acquisition			Jan. 30, 2014
Payments to Acquire Businesses			$ 26.9
Number of Wind Energy Facilities Acquired			3
Assets [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 [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			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
Expected Tax Deductible Amount of Goodwill			0.0
Power Purchase Agreements, Current			$ 12.4
Non-Controlling Interest, Decrease from Purchase of Interests		$ 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

[1] Power Purchase Agreements were acquired in conjunction with ALLETE Clean Energy’s wind energy facilities acquisitions. (See Note 4. Acquisitions.)

[2] Power Purchase Agreements were acquired in conjunction with ALLETE Clean Energy’s wind energy facilities acquisitions. (See Note 4. 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 relating to Lake Benton, Storm Lake II and Condon at fair value using the discounted cash flow method.