Acquisitions - ACE Wind Acquisition (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	12 Months Ended		0 Months Ended		3 Months Ended	12 Months Ended
	Feb. 11, 2014	Dec. 31, 2014		Jan. 30, 2014		Mar. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Business Combination, Reason for Business Combination		The acquisitions below are consistent with ALLETE’s stated strategy of investing in energy infrastructure and related services to complement its core regulated utility, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth.
Liabilities [Abstract]
Long-Term Debt Due Within one Year				$ 2.2
Power Purchase Agreements		110.7	[1]				0	[1]
Power Purchase Agreements - Current		19.4	[1]				0	[1]
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	6.0
Armenia Mountain [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity		Armenia Mountain
Generating Capacity (MW)		101.0
Liabilities [Abstract]
Purchase option				0.3
ACE Wind [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition						Jan. 30, 2014
Business Acquisition, Transaction Costs				26.9
Assets [Abstract]
Cash and Cash Equivalents				3.8
Other Current Assets				14.3
Property, Plant and Equipment - Net				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
Power Purchase Agreements - Current				12.4
Business Combination, Acquisition Related Costs		$ 1.4
Business Acquisition, Pro Forma Information, Description		not material					not materia
Storm Lake II [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity		Storm Lake II
Generating Capacity (MW)		77
Lake Benton [Member]
Business Acquisition [Line Items]
Business Acquisition, Name of Acquired Entity		Lake Benton
Generating Capacity (MW)		104.0
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 7. 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 related 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.