Acquisitions - Storm Lake I (Details) $ in Millions		Dec. 17, 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
Storm Lake I [Member]
Business Acquisition [Line Items]
Date of Acquisition		Dec. 17, 2014
Name of Acquired Entity		Storm Lake I
Payments to Acquire Businesses		$ 15.1
Generating Capacity (MW) | MW		108
Assets [Abstract]
Cash and Cash Equivalents		$ 0.4
Other Current Assets		4.7
Property, Plant and Equipment		47.3
Other Non-Current Assets	[3]	11.4
Total Assets Acquired		63.8
Liabilities [Abstract]
Current Liabilities	[4]	8.2
Power Purchase Agreements, Non-Current		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, Noncurrent		$ 0.4
Goodwill		immaterial
Expected Tax Deductible Amount of Goodwill		$ 0.0
Power Purchase Agreements, Current		$ 7.5

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

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