Acquisitions - Wind Development Project (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended	3 Months Ended		0 Months Ended
	Nov. 20, 2014	Dec. 31, 2014		Apr. 15, 2015	Mar. 31, 2015
Assets [Abstract]
Construction Costs for Development Project		$ 48.2	[1]		$ 48.4	[1]
Liabilities [Abstract]
Customer Deposits Received for Development Project		54.3	[2]		54.3	[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
Contractual Sales Price	200
Assets [Abstract]
Cash and Cash Equivalents	3.6
Construction Costs for Development Project	23.4	48.2
Goodwill	0
Liabilities [Abstract]
Customer Deposits Received for Development Project		54.3			54.3
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
Subsequent Event [Member] | Wind Development Project [Member]
Liabilities [Abstract]
Increase (Decrease) in Customer Advances				$ 50

[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. (See 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. (See Prepayments and Other Current Assets table and Note 4. Acquisitions.)