Acquisitions - Wind Development Project (Details) $ in Millions			6 Months Ended
		Nov. 20, 2014 USD ($) MW	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Assets [Abstract]
Construction Costs	[1]		$ 0.0	$ 48.2
Billings in Excess of Costs and Estimated Earnings [Abstract]
Contract Billings	[2]		0.0	54.3
Construction Costs	[1]		0.0	48.2
Billings in Excess of Cost and Estimated Earnings	[3]		$ 54.4	0.0
Wind Development Project [Member]
Business Acquisition [Line Items]
Date of Acquisition		Nov. 20, 2014
Payments to Acquire Businesses		$ 27.0
Generating Capacity (MW) | MW		107
Number of Wind Turbines			43
Contractual Sales Price		$ 200.0
Assets [Abstract]
Cash and Cash Equivalents		3.6
Construction Costs		23.4	$ 84.3	48.2
Goodwill		0.0
Liabilities [Abstract]
Total Liabilities Assumed		0.0
Billings in Excess of Costs and Estimated Earnings [Abstract]
Contract Billings			141.2	54.3
Construction Costs		$ 23.4	84.3	$ 48.2
Estimated Earnings			2.5
Billings in Excess of Cost and Estimated Earnings	[4]		54.4
Revenue			20.5
Cost of Sales			$ 18.0

[1] Construction Costs for Development Project relate to ALLETE Clean Energy’s acquisition in November 2014 of a project to develop and construct a wind energy facility in 2015. As of June 30, 2015, these costs have been netted with contract billings. (See Billings in Excess of Costs and Estimated Earnings in Other Current Liabilities table and Note 4. Acquisitions.)

[2] Construction Deposits Received for Development Project relate to ALLETE Clean Energy’s project to develop and construct a wind energy facility in 2015. As of June 30, 2015, these deposits have been netted with contract costs and estimated gross profit. (See Billings in Excess of Costs and Estimated Earnings below and Note 4. Acquisitions.)

[3] Billings in Excess of Costs and Estimated Earnings represents the excess of contract billings over the construction costs incurred and estimated earnings recognized. In the second quarter of 2015, the NDPSC approved the sale agreement ALLETE Clean Energy has with Montana-Dakota Utilities to develop, construct, and sell a wind energy facility in 2015. (See Note 4. Acquisitions.)

[4] Included in Other Current Liabilities on the Consolidated Balance Sheet.