Acquisitions - Armenia Mountain (Details) $ in Millions		1 Months Ended	12 Months Ended
		Jul. 31, 2015 USD ($) MW	Dec. 31, 2015 USD ($)	Mar. 31, 2016 USD ($)
Liabilities Assumed [Abstract]
Restricted Cash - Current	[1]		$ 5.6	$ 11.5
Restricted Cash - Non-Current			8.1	$ 8.1
Armenia Mountain [Member]
Business Acquisition [Line Items]
Date of Acquisition		July 2015
Percentage of Voting Interests Acquired		100.00%
Name of Acquired Entity		Armenia Mountain
Payments to Acquire Business		$ 111.1
Assets Acquired [Abstract]
Current Assets	[2]	9.0
Property, Plant and Equipment		156.2
Other Non-Current Assets	[3]	14.4
Total Assets Acquired		179.6
Liabilities Assumed [Abstract]
Current Liabilities		2.9
Long-Term Debt Due Within One Year		5.9
Long-Term Debt		55.0
Other Non-Current Liabilities		4.7
Total Liabilities Assumed		68.5
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		immaterial
Tax Deductible Goodwill		$ 0.0
Acquisition Related Costs			$ 1.6
Armenia Mountain [Member] | Armenia Mountain PPAs (expire 2024) [Member]
Business Acquisition [Line Items]
Generating Capacity (MW) | MW		100.5

[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 Current Assets was $1.0 million related to the current portion of Power Purchase Agreements and $6.0 million of restricted cash related to collateral deposits required under its loan agreement.

[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 collateral deposits required under its loan agreements, and an immaterial amount of goodwill. For tax purposes, the purchase price allocation resulted in no allocation to goodwill.