Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2015		Dec. 31, 2014
Business Segments [Line Items]
Total Inventories		$ 107.8		$ 80.5
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause		11.8		16.3
Construction Costs for Development Project	[1]	0.0		48.2
Restricted Cash	[2]	7.7		2.7
Other		17.0		14.8
Total Prepayments and Other Current Assets		36.5		82.0
Restricted Cash and Investments, Noncurrent		20.3		5.3
Other Current Liabilites [Abstract]
Customer Deposits		18.5		19.7
Power Purchase Agreements	[3]	24.3		19.4
Construction Deposits Received for Development Project	[4]	0.0		54.3
Billings in Excess of Cost and Estimated Earnings	[5]	54.4		0.0
Other		38.4		27.4
Total Other Current Liabilities		135.6		120.8
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		122.4		109.2
Power Purchase Agreements	[6]	149.5		110.7
Contingent Consideration	[7]	36.8		0.0
Other		45.2		45.1
Total Other Non-Current Liabilities		353.9		265.0
Regulated Operations [Member]
Business Segments [Line Items]
Fuel		43.7		29.0
Materials and Supplies		34.2		35.2
Total Inventories		77.9		64.2
Investments and Other [Member]
Business Segments [Line Items]
Materials and Supplies		16.6		16.3
Raw Materials		2.9	[8]	0.0
Work in Progress		1.5	[8]	0.0
Finished Goods		9.1	[8]	0.0
Reserve for Obsolescence		(0.2)	[8]	0.0
Total Inventories		29.9		$ 16.3
Restricted Cash Held in Escrow [Member]
Restricted Cash and Investments, Noncurrent		15.0
Restricted Cash Related to Debt Service and Other Requirements [Member]
Restricted Cash and Investments, Noncurrent		$ 5.3
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Definite-Lived Intangible Assets, Useful Life (Years)		1 year
Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Definite-Lived Intangible Assets, Useful Life (Years)		23 years

[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] Restricted Cash related to ALLETE Clean Energy’s wind energy facilities operating expense and capital distribution reserve requirements and cash pledged as collateral by U.S. Water Services for stand-by letters of credit.

[3] Power Purchase Agreements were acquired in conjunction with ALLETE Clean Energy’s wind energy facilities acquisitions. (See Note 4. Acquisitions.)

[4] 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.)

[5] 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.)

[6] Power Purchase Agreements were acquired in conjunction with ALLETE Clean Energy’s wind energy facilities acquisitions. (See Note 4. Acquisitions.)

[7] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 4. Acquisitions and Note 7. Fair Value.)

[8] Raw Materials, Work in Progress, Finished Goods, and Reserve for Obsolescence presented relate to U.S. Water Services which was acquired on February 10, 2015.