Acquisitions - Storm Lake I (Details) $ in Millions		1 Months Ended	12 Months Ended
		Dec. 31, 2014 USD ($) MW	Dec. 31, 2014 USD ($) MW	Dec. 31, 2015 USD ($)
Liabilities Assumed [Abstract]
Power Purchase Agreements - Non-Current Liability	[1]	$ 110.7	$ 110.7	$ 138.1
Restricted Cash - Non-Current		5.3	5.3	8.1
Power Purchase Agreements - Current Liability	[2]	$ 19.4	$ 19.4	$ 23.3
Storm Lake I [Member]
Business Acquisition [Line Items]
Date of Acquisition		December 2014
Name of Acquired Entity		Storm Lake I
Payments to Acquire Business		$ 15.1
Generating Capacity (MW) | MW		108	108
Assets Acquired [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	[3]	11.4	11.4
Total Assets Acquired		63.8	63.8
Liabilities Assumed [Abstract]
Current Liabilities	[4]	8.2	8.2
Power Purchase Agreements - Non-Current Liability		23.5	23.5
Non-Current Liabilities		17.0	17.0
Total Liabilities Assumed		48.7	48.7
Net Identifiable Assets Acquired		15.1	15.1
Restricted Cash - Non-Current		$ 0.4	0.4
Goodwill		immaterial
Deductible Goodwill		$ 0.0	0.0
Power Purchase Agreements - Current Liability		$ 7.5	$ 7.5
Acquisition-Related Costs			immaterial

[1] Power Purchase Agreements acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions. (See Note 7. Acquisitions.)

[2] Power Purchase Agreements acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions. (See Note 7. 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.