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Business Segments (Details) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 USD ($) a | Sep. 30, 2016 USD ($) | Sep. 30, 2017 USD ($) a Segments | Sep. 30, 2016 USD ($) | Dec. 31, 2016 USD ($)
Business Segments [Line Items] |  |  |  |  |
Number of Reportable Segments | Segments |  |  | 3 |  |
Operating Revenue | $ 362.5 | $ 349.6 | $ 1,081.4 | $ 998.2 |
Net Income (Loss) Attributable to ALLETE | 44.9 | 40.3 | 130.8 | 111.0 |
Assets | 4,978.5 |  | $ 4,978.5 |  | $ 4,876.9
Regulated Operations [Member] |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |
Number of Operating Segments | Segments |  |  | 3 |  |
Operating Revenue | 277.6 | 253.3 | $ 824.1 | 740.5 |
Net Income (Loss) Attributable to ALLETE | 34.2 | 45.0 | 110.1 | 110.0 |
Assets | 3,811.7 |  | 3,811.7 |  | 3,823.9
ALLETE Clean Energy [Member] |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |
Operating Revenue | 13.5 | 14.7 | 56.8 | 57.1 |
Net Income (Loss) Attributable to ALLETE | 0.6 | 1.0 | 11.1 | 9.7 |
Assets | 560.3 |  | 560.3 |  | 566.0
U.S. Water Services [Member] |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |
Operating Revenue | 40.2 | 37.8 | 110.7 | 104.5 |
Net Income (Loss) Attributable to ALLETE | 1.3 | 1.5 | 1.6 | 2.0 |
Assets | $ 293.3 |  | $ 293.3 |  | 264.1
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 | $ 31.2 | 43.8 | $ 89.8 | 96.1 |
Net Income (Loss) Attributable to ALLETE | 8.8 | $ (7.2) | 8.0 | $ (10.7) |
Assets | $ 313.2 |  | $ 313.2 |  | $ 222.9

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