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Business Segments (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2017 USD ($) a Segments | Mar. 31, 2016 USD ($) | Dec. 31, 2016 USD ($)
Business Segments [Line Items] |  |  |
Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments | We present three reportable segments: Regulated Operations, ALLETE Clean Energy and U.S. Water Services. We measure performance of our operations through budgeting and monitoring of contributions to consolidated net income by each business segment. |  |
Number of Reportable Segments | 3 |  |
Operating Revenue | $ 365.6 | $ 333.8 |
Net Income (Loss) Attributable to ALLETE | 49.0 | 45.9 |
Assets | $ 4,941.9 |  | $ 4,876.9
Regulated Operations [Member] |  |  |
Business Segments [Line Items] |  |  |
Number of Operating Segments | Segments | 3 |  |
Operating Revenue | $ 281.6 | 252.3 |
Net Income (Loss) Attributable to ALLETE | 43.5 | 42.4 |
Assets | 3,828.2 |  | 3,823.9
ALLETE Clean Energy [Member] |  |  |
Business Segments [Line Items] |  |  |
Operating Revenue | 23.7 | 23.6 |
Net Income (Loss) Attributable to ALLETE | 6.7 | 6.1 |
Assets | 571.7 |  | 566.0
U.S. Water Services [Member] |  |  |
Business Segments [Line Items] |  |  |
Operating Revenue | 32.1 | 32.4 |
Net Income (Loss) Attributable to ALLETE | (0.3) | (0.5) |
Assets | $ 265.1 |  | 264.1
Corporate and Other [Member] |  |  |
Business Segments [Line Items] |  |  |
Number of Operating Segments | Segments | 2 |  |
Land in Minnesota (Acres) | a | 5,000 |  |
Operating Revenue | $ 28.2 | 25.5 |
Net Income (Loss) Attributable to ALLETE | (0.9) | $ (2.1) |
Assets | $ 276.9 |  | $ 222.9

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