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Business Segments (Details) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2016 USD ($) a | Sep. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) a Segments | Sep. 30, 2015 USD ($) | Dec. 31, 2015 USD ($)
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue |  | $ 349.6 | $ 462.5 | $ 998.2 | $ 1,105.8 |
Net Income (Loss) Attributable to ALLETE |  | 40.3 | 60.4 | 111.0 | 122.8 |
Assets | [1] | 4,874.8 |  | $ 4,874.8 |  | $ 4,894.5
Regulated Operations [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Number of Operating Segments | Segments |  |  |  | 3 |  |
Operating Revenue |  | 253.3 | 250.2 | $ 740.5 | 743.0 |
Net Income (Loss) Attributable to ALLETE |  | 45.0 | 43.8 | 110.0 | 108.1 |
Assets | [1] | 3,838.8 |  | 3,838.8 |  | 3,853.1
ALLETE Clean Energy [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue |  | 14.7 | 151.1 | 57.1 | 197.5 |
Net Income (Loss) Attributable to ALLETE |  | 1.0 | 13.2 | 9.7 | 18.7 |
Assets | [1] | 483.5 |  | 483.5 |  | 501.5
U.S. Water Services [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue |  | 37.8 | 36.1 | 104.5 | 86.0 |
Net Income (Loss) Attributable to ALLETE |  | 1.5 | 1.0 | 2.0 | 1.5 |
Assets |  | $ 259.4 |  | $ 259.4 |  | 258.3
Corporate and Other [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Number of Operating Segments | Segments |  |  |  | 2 |  |
Land in Minnesota (Acres) | a |  | 5,000 |  | 5,000 |  |
Operating Revenue |  | $ 43.8 | 25.1 | $ 96.1 | 79.3 |
Net Income (Loss) Attributable to ALLETE |  | (7.2) | $ 2.4 | (10.7) | $ (5.5) |
Assets |  | $ 293.1 |  | $ 293.1 |  | $ 281.6
[1] | As a result of revised accounting guidance adopted in the first quarter of 2016, we reclassified unamortized debt issuance costs from Other Non-Current Assets to Long-Term Debt on the Consolidated Balance Sheet. Prior period segment assets have been revised to conform to the current presentation. (See Note 1. Operations and Significant Accounting Policies.)

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