Acquisitions - Chanarambie/Viking (Details) $ in Millions			6 Months Ended
		Apr. 15, 2015 USD ($) MW	Jun. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Liabilities [Abstract]
Power Purchase Agreements, Non-Current Liability	[1]			$ 143.9	$ 110.7
Goodwill				131.0	2.9
Power Purchase Agreements, Current Liability	[2]			$ 23.8	$ 19.4
Chanarambie/Viking [Member]
Business Acquisition [Line Items]
Date of Acquisition		Apr. 15, 2015
Name of Acquired Entity		Chanarambie/Viking
Payments to Acquire Business		$ 48.0
Generating Capacity (MW) | MW		97.5
Assets [Abstract]
Current Assets		$ 4.8
Property, Plant and Equipment		103.0
Other Non-Current Assets	[3]	1.0
Total Assets Acquired		108.8
Liabilities [Abstract]
Current Liabilities	[4]	6.7
Power Purchase Agreements, Non-Current Liability		49.0
Non-Current Liabilities		5.1
Total Liabilities Assumed		60.8
Net Identifiable Assets Acquired		48.0
Goodwill		0.3
Expected Tax Deductible Amount of Goodwill		0.0
Power Purchase Agreements, Current Liability		$ 5.9
Acquisition Related Costs			$ 0.2
Chanarambie/Viking [Member] | Chanarambie/Viking PPA (expires 2018) [Member]
Business Acquisition [Line Items]
Generating Capacity (MW) | MW		12
Chanarambie/Viking [Member] | Chanarambie/Viking PPA (expires 2023) [Member]
Business Acquisition [Line Items]
Generating Capacity (MW) | MW		85.5

[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 $0.3 million of goodwill; for tax purposes, the purchase price allocation resulted in no allocation to goodwill.

[4] Current Liabilities included $5.9 million related to the current portion of Power Purchase Agreements.