Acquisitions - Storm Lake I (Details) $ in Millions		12 Months Ended
		Dec. 31, 2014 USD ($) MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Liabilities Assumed [Abstract]
Power Sales Agreements - Non-Current Liability			$ 113.8	$ 138.1
Restricted Cash - Non-Current	[1]		8.6	8.1
Power Sales Agreements - Current Liability			$ 24.6	$ 23.3
Storm Lake I [Member]
Business Acquisition [Line Items]
Name of Acquired Entity		Storm Lake I
Payments to Acquire Business		$ 15.1
Generating Capacity (MW) | MW		108
Assets Acquired [Abstract]
Cash and Cash Equivalents		$ 0.4
Other Current Assets		4.7
Property, Plant and Equipment		47.3
Other Non-Current Assets	[2]	11.4
Total Assets Acquired		63.8
Liabilities Assumed [Abstract]
Current Liabilities	[3]	8.2
Power Sales Agreements - Non-Current Liability		23.5
Non-Current Liabilities		17.0
Total Liabilities Assumed		48.7
Net Identifiable Assets Acquired		15.1
Restricted Cash - Non-Current		0.4
Tax Deductible Goodwill		0.0
Power Sales Agreements - Current Liability		$ 7.5

[1] Restricted Cash includes collateral deposits required under ALLETE Clean Energy’s loan agreements and PSAs, and deposits from SWL&P customers in aid of future capital expenditures.

[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] Current Liabilities included $7.5 million related to the current portion of PSAs.