Acquisitions - Armenia Mountain (Details) $ in Millions			9 Months Ended
		Jul. 01, 2015 USD ($) MW	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Liabilities [Abstract]
Restricted Cash, Current	[1]		$ 8.1	$ 2.7
Goodwill			131.0	$ 2.9
Armenia Mountain [Member]
Business Acquisition [Line Items]
Date of Acquisition		Jul. 01, 2015
Percentage of Voting Interests Acquired		100.00%
Name of Acquired Entity		Armenia Mountain
Payments to Acquire Business		$ 111.1
Generating Capacity (MW) | MW		100.5
Assets [Abstract]
Current Assets	[2]	$ 9.0
Property, Plant and Equipment		156.7
Other Non-Current Assets	[3]	14.4
Total Assets Acquired		180.1
Liabilities [Abstract]
Current Liabilities		3.4
Long-term Debt Due Within One Year		5.9
Long-term Debt		55.0
Other Non-Current Liabilities		4.7
Total Liabilities Assumed		69.0
Net Identifiable Assets Acquired		111.1
Power Purchase Agreements, Current Asset		1.0
Restricted Cash, Current		6.0
Power Purchase Agreements, Non-Current Asset		8.2
Restricted Cash, Non-Current		6.1
Goodwill		0.1
Expected Tax Deductible Amount of Goodwill		$ 0.0
Acquisition Related Costs			$ 1.6

[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 Current Assets was $1.0 million related to the current portion of Power Purchase Agreements and $6.0 million of restricted cash related to capital distribution reserve requirements.

[3] Included in Other Non-Current Assets was $8.2 million related to the non-current portion of Power Purchase Agreements, $6.1 million of restricted cash related to operating expense and major maintenance reserve requirements, and $0.1 million of goodwill; for tax purposes, the purchase price allocation resulted in no allocation to goodwill.