Acquisitions - Wind Construction Project (Details) $ in Millions		1 Months Ended	3 Months Ended								12 Months Ended
		Nov. 30, 2014 USD ($) MW	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Revenue			$ 380.6	$ 462.5	$ 323.3	$ 320.0	$ 290.7	$ 288.9	$ 260.7	$ 296.5	$ 1,486.4	$ 1,136.8	$ 1,018.4
Assets Acquired [Abstract]
Construction Work in Progress	[1]		0.0				48.2				0.0	48.2
Goodwill			130.6				2.9				130.6	2.9
Liabilities Assumed [Abstract]
Cost of Sales											302.3	77.9	$ 71.2
Contract Billings	[2]		$ 0.0				54.3				0.0	54.3
Wind Construction Project [Member]
Business Acquisition [Line Items]
Date of Acquisition		November 2014
Payments to Acquire Business		$ 27.0
Generating Capacity (MW) | MW		107
Number of Wind Turbines		43
Revenue											197.7
Assets Acquired [Abstract]
Cash and Cash Equivalents		$ 3.6
Construction Work in Progress		23.4					48.2					48.2
Goodwill		0.0
Liabilities Assumed [Abstract]
Total Liabilities Assumed		$ 0.0
Cost of Sales											$ 162.9
Contract Billings							$ 54.3					$ 54.3

[1] Construction Costs for Development Project related to ALLETE Clean Energy’s project to construct a wind energy facility which was sold in 2015. (See Note 7. Acquisitions.)

[2] Construction Deposits Received for Development Project relate to ALLETE Clean Energy’s project to construct a wind energy facility which was sold in 2015. (See Note 7. Acquisitions.)