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Acquisitions - Wind Construction Project (Details) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Nov. 30, 2014 USD ($) MW | Dec. 31, 2015 USD ($) | Sep. 30, 2015 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Sep. 30, 2014 USD ($) | Jun. 30, 2014 USD ($) | Mar. 31, 2014 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  | $ 380.6 | $ 462.5 | $ 323.3 | $ 320.0 | $ 290.7 | $ 288.9 | $ 260.7 | $ 296.5 | $ 1,486.4 | $ 1,136.8 | $ 1,018.4
Assets Acquired [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Construction Work in Progress | [1] |  | 0.0 |  |  |  | 48.2 |  |  |  | 0.0 | 48.2 |
Goodwill |  |  | 130.6 |  |  |  | 2.9 |  |  |  | 130.6 | 2.9 |
Liabilities Assumed [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Cost of Sales |  |  |  |  |  |  |  |  |  |  | 302.3 | 77.9 | $ 71.2
Contract Billings | [2] |  | $ 0.0 |  |  |  | 54.3 |  |  |  | 0.0 | 54.3 |
Wind Construction Project [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Date of Acquisition |  | November 2014 |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Business |  | $ 27.0 |  |  |  |  |  |  |  |  |  |  |
Generating Capacity (MW) | MW |  | 107 |  |  |  |  |  |  |  |  |  |  |
Number of Wind Turbines |  | 43 |  |  |  |  |  |  |  |  |  |  |
Revenue |  |  |  |  |  |  |  |  |  |  | 197.7 |  |
Assets Acquired [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Cash and Cash Equivalents |  | $ 3.6 |  |  |  |  |  |  |  |  |  |  |
Construction Work in Progress |  | 23.4 |  |  |  |  | 48.2 |  |  |  |  | 48.2 |
Goodwill |  | 0.0 |  |  |  |  |  |  |  |  |  |  |
Liabilities Assumed [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Total Liabilities Assumed |  | $ 0.0 |  |  |  |  |  |  |  |  |  |  |
Cost of Sales |  |  |  |  |  |  |  |  |  |  | $ 162.9 |  |
Contract Billings |  |  |  |  |  |  | $ 54.3 |  |  |  |  | $ 54.3 |
[1] | Construction Costs for Development Project related to ALLETE Clean Energy’s project to construct a wind energy facility which was sold in 2015. (See Note 7. Acquisitions.)
[2] | Construction Deposits Received for Development Project relate to ALLETE Clean Energy’s project to construct a wind energy facility which was sold in 2015. (See Note 7. Acquisitions.)

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