Acquisitions - Storm Lake I (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended		3 Months Ended
	Dec. 17, 2014		Mar. 31, 2015 MW		Dec. 31, 2014		Dec. 17, 2014
Liabilities [Abstract]
Power Purchase Agreements			106.3	[1]	110.7	[1]
Power Purchase Agreement - Current			18.9	[1]	19.4	[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	15.1
Generating Capacity (MW)			108.0
Assets [Abstract]
Cash and Cash Equivalents	0.4						0.4
Other Current Assets	4.7						4.7
Property, Plant and Equipment	47.3						47.3
Other Non-Current Assets	11.4	[2]					11.4	[2]
Total Assets Acquired	63.8						63.8
Liabilities [Abstract]
Other Current Liabilities	8.2	[3]					8.2	[3]
Power Purchase Agreements	23.5						23.5
Other Non-Current Liabilities	17.0						17.0
Total Liabilities and Non-Controlling Interest Assumed	48.7						48.7
Net Identifiable Assets Acquired	15.1						15.1
Restricted Cash and Investments, Noncurrent	0.4						0.4
Goodwill Description	immaterial
Power Purchase Agreement - Current	$ 7.5						$ 7.5

[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 $0.4 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.