Acquisitions - U.S. Water Services (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended	6 Months Ended	11 Months Ended
		Feb. 28, 2015	Mar. 31, 2015	Jun. 30, 2016	Dec. 31, 2015
Assets Acquired [Abstract]
Goodwill				$ 130.6	$ 130.6
Liabilities Assumed [Abstract]
Restricted Cash - Current	[1]			$ 7.5	$ 5.6
U.S. Water Services [Member]
Business Acquisition [Line Items]
Name of Acquired Entity		U.S. Water Services
Total Consideration		$ 202.3
Payments to Acquire Business		166.6
Contingent Consideration		$ 35.7
Percent of Results of Operations Reflected in Income Statement				100.00%	100.00%
Percentage of Voting Interests Acquired		100.00%
Assets Acquired [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
Intangible Assets	[4]	83.0
Goodwill	[5]	122.9
Other Non-Current Assets		0.2
Total Assets Acquired		253.1
Liabilities Assumed [Abstract]
Current Liabilities		19.2
Non-Current Liabilities		31.6
Total Liabilities Assumed		50.8
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
Tax Deductible Goodwill		$ 2.9
Acquisition Related Costs			$ 3.0

[1] Restricted Cash includes collateral deposits required under ALLETE Clean Energy’s loan agreements and cash pledged as collateral for U.S. Water Services’ standby 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 were 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 was 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 standby letters of credit.

[4] Intangible Assets include customer relationships, patents, non-compete agreements, and trademarks and trade names. (See Note 4. Goodwill and Intangible Assets.)

[5] For tax purposes, the purchase price allocation resulted in $2.9 million of deductible goodwill.