Acquisitions - U.S. Water Services (Details) - USD ($) $ in Millions			3 Months Ended	6 Months Ended
		Feb. 10, 2015	Mar. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Liabilities [Abstract]
Restricted Cash, Current	[1]			$ 7.7	$ 2.7
U.S. Water Services [Member]
Business Acquisition [Line Items]
Date of Acquisition		Feb. 10, 2015
Name of Acquired Entity		U.S. Water Services
Consideration Transferred		$ 202.3
Payments to Acquire Businesses		166.6
Contingent Consideration		$ 35.7
Percent of Results of Operations Reflected in Income Statement				100.00%
Percentage of Voting Interests Acquired		100.00%
Assets [Abstract]
Cash and Cash Equivalents		$ 0.9
Accounts Receivable		16.8
Inventories	[2]	13.4
Other Current Assets	[3]	5.3
Property, Plant and Equipment		10.6
Goodwill	[4]	127.1
Intangible Assets	[5]	83.0
Other Non-Current Assets		0.2
Total Assets Acquired		257.3
Liabilities [Abstract]
Current Liabilities		18.7
Non-Current Liabilities		36.3
Total Liabilities and Non-Controlling Interest Assumed		55.0
Net Identifiable Assets Acquired		202.3
Fair Value Adjustments for Work in Process and Finished Goods		2.7
Fair Value of Sales Backlog		1.6
Restricted Cash, Current		2.1
Expected Tax Deductible Amount of Goodwill		$ 3.2
Acquisition Related Costs			$ 3.0

[1] 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.

[2] Included in Inventories was $2.7 million of fair value adjustments relating to work in progress and finished goods inventories which will be recognized as Cost of Sales within one year from the acquisition date.

[3] Included in Other Current Assets was $1.6 million relating to the fair value of sales backlog. Sales backlog will be recognized as Cost of Sales within one year from the acquisition date. Also included in Other Current Assets was restricted cash of $2.1 million relating to cash pledged as collateral for stand-by letters of credit.

[4] For tax purposes, the purchase price allocation resulted in $3.2 million of deductible Goodwill.

[5] Intangible Assets include customer relationships, patents, non-compete agreements and trademarks and trade names. (See Note 5. Goodwill and Intangible Assets.)