Operations and Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Business Segments [Line Items]
Total Inventories	102.7		$ 80.5
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause	15.6		16.3
Construction Costs for Development Project	48.4	[1]	48.2	[1]
Restricted Cash	5.6	[2]	2.7	[2]
Other	18.9		14.8
Total Prepayments and Other Current Assets	88.5		82.0
Other Non-Current Assets [Abstract]
Restricted Cash	5.3		5.3
Other Current Liabilites [Abstract]
Customer Deposits	20.0		19.7
Power Purchase Agreements	18.9	[3]	19.4	[3]
Construction Deposits Received for Development Project	54.3	[4]	54.3	[4]
Other	25.4		27.4
Total Other Current Liabilities	118.6		120.8
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation	111.6		109.2
Power Purchase Agreements	106.3	[3]	110.7	[3]
Contingent Consideration	35.8	[5]	0	[5]
Other	48.6		45.1
Total Other Non-Current Liabilities	302.3		265.0
Minimum [Member]
Goodwill and Intangible Assets [Abstract]
Estimated Useful Life	1 year
Maximum [Member]
Goodwill and Intangible Assets [Abstract]
Estimated Useful Life	23 years
Regulated Operations [Member]
Business Segments [Line Items]
Fuel	38.0		29.0
Materials and Supplies	34.6		35.2
Total Inventories	72.6		64.2
Investments and Other [Member]
Business Segments [Line Items]
Materials and Supplies	16.3		16.3
Raw Materials	3.1	[6]	0
Work in Progress	1.0	[6]	0
Finished Goods	10.2	[6]	0
Reserve for Obsolescence	(0.5)	[6]	0
Total Inventories	30.1		$ 16.3

[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] 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 the ALLETE Clean Energy wind energy facilities acquisitions in 2014. (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. (See Prepayments and Other Current Assets table and Note 4. Acquisitions.)

[5] Contingent Consideration relates to the estimated fair value of the earnings-based payment to acquire the remaining ownership interest in U.S. Water Services. (See Note 4. Acquisitions and Note 7. Fair Value.)

[6] Raw materials, Work in Progress, Finished Goods, and Reserve for Obsolescence presented are attributable to U.S. Water Services which was acquired February 10, 2015.