Acquisitions - Wind Development Project (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended		12 Months Ended
	Nov. 20, 2014	Dec. 31, 2014 MW		Dec. 31, 2014 MW		Dec. 31, 2013		Nov. 20, 2014
Assets [Abstract]
Construction in Progress, Gross		$ 48.2	[1]	$ 48.2	[1]	0	[1]
Liabilities [Abstract]
Deposits		54.3	[2]	54.3	[2]	0	[2]
Wind Development Project [Member]
Business Acquisition [Line Items]
Business Acquisition, Effective Date of Acquisition		Nov. 20, 2014
Payments to Acquire Businesses, Gross	27.0
Generating Capacity (MW)		107.0		107.0
Contractual Sales Price	200
Assets [Abstract]
Cash and Cash Equivalents	3.6							3.6
Construction in Progress, Gross	23.4	48.2		48.2				23.4
Goodwill	0							0
Liabilities [Abstract]
Total Liabilities and Non-Controlling Interest Assumed	0							0
Business Combination, Acquisition Related Cost				not material
Business Acquisition, Pro Forma Information, Description				not material		not material
Deposits		$ 54.3		$ 54.3

[1] Construction Costs for Development Project acquired/incurred due to ALLETE Clean Energy’s acquisition on November 20, 2014 of a project to develop and construct a wind energy facility in 2015. (See Other Current Liabilities table and Note 7. Acquisitions.)

[2] Construction Deposits Received for Development Project are due to ALLETE Clean Energy’s project to develop and construct a wind energy facility in 2015. (See Prepayment and Other Current Assets table and Note 7. Acquisitions.)