Acquisitions - Storm Lake I (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended		3 Months Ended		12 Months Ended
	Dec. 17, 2014		Dec. 31, 2014 MW		Dec. 31, 2014 MW		Dec. 31, 2013
Liabilities [Abstract]
Power Purchase Agreements			110.7	[1]	110.7	[1]	$ 0	[1]
Power Purchase Agreements - Current			19.4	[1]	19.4	[1]	0	[1]
Storm Lake I [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition			Dec. 17, 2014
Business Acquisition, Name of Acquired Entity					Storm Lake I
Payments to Acquire Businesses, Gross	15.0
Generating Capacity (MW)			108.0		108.0
Assets [Abstract]
Cash and Cash Equivalents	0.4
Other Current Assets	4.7
Property, Plant and Equipment - Net	47.3
Other Non-Current Assets	11.4	[2]
Total Assets Acquired	63.8
Liabilities [Abstract]
Other Current Liabilities	8.2	[3]
Power Purchase Agreements	23.5
Other Non-Current Liabilities	17.0
Total Liabilities and Non-Controlling Interest Assumed	48.7
Net Identifiable Assets Acquired	15.1
Restricted Cash and Investments, Noncurrent	0.4
Goodwill Description	immaterial
Power Purchase Agreements - Current	$ 7.5
Business Combination, Acquisition Related Cost					not material
Business Acquisition, Pro Forma Information, Description					not material

[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 $0.4 million of restricted cash and an immaterial amount of goodwill; for tax purposes, the purchase price allocation resulted in no allocation to goodwill.

[3] Other Current Liabilities included $7.5 million related to the current liabilities portion of the Power Purchase Agreements.