Acquisitions - ACE Wind Acquisition (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended			3 Months Ended
	Feb. 11, 2014	Jan. 30, 2014		Mar. 31, 2014	Mar. 31, 2015 MW		Dec. 31, 2014
Liabilities [Abstract]
Power Purchase Agreements					106.3	[1]	$ 110.7	[1]
Power Purchase Agreements					18.9	[1]	19.4	[1]
Non-Controlling Interest, Decrease from Purchase of Interests	6.0
Armenia Mountain [Member]
Liabilities [Abstract]
Purchase option		0.3
ACE Wind [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition				Jan. 30, 2014
Payments to Acquire Businesses, Gross		26.9
Assets [Abstract]
Cash and Cash Equivalents		3.8
Other Current Assets		14.3
Property, Plant and Equipment		156.9
Other Non-Current Assets		7.5	[2]
Total Assets Acquired		182.5
Liabilities [Abstract]
Other Current Liabilities		15.2	[3]
Long-term Debt Due Within One Year		2.2
Long-term Debt		21.1
Power Purchase Agreements		99.4
Other Non-Current Liabilities		10.6
Non-Controlling Interest		7.1	[4]
Total Liabilities and Non-Controlling Interest Assumed		155.6
Net Identifiable Assets Acquired		26.9
Goodwill		2.9
Business Acquisition, Goodwill, Expected Tax Deductible Amount		0
Power Purchase Agreements		$ 12.4
Lake Benton [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity					Lake Benton
Generating Capacity (MW)					104.0
Storm Lake II [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity					Storm Lake II
Generating Capacity (MW)					77
Condon [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity					Condon
Generating Capacity (MW)					50

[1] Power Purchase Agreements were acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions in 2014. (See Note 4. Acquisitions.)

[2] Included in Other Non-Current Assets was $0.3 million for the option to purchase Armenia Mountain in 2015, and goodwill of $2.9 million; for tax purposes, the purchase price allocation resulted in no allocation to goodwill.

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

[4] 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. The non-controlling interest related to Lake Benton and Storm Lake II was subsequently purchased by ALLETE Clean Energy.