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Business Segments (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 USD ($) a Segments | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($)
Business Segments [Line Items] |  |  |  |
Number of Reportable Segments | Segments |  | 3 |  |
Operating Revenue |  | $ 358.2 | $ 365.6 |
Net Income (Loss) |  | 51.0 | 49.0 |
Assets |  | $ 5,072.1 |  | $ 5,080.0
Regulated Operations [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Number of Operating Segments | Segments |  | 3 |  |
Operating Revenue |  | $ 270.2 | 281.6 |
Net Income (Loss) |  | 43.9 | 43.5 |
Assets |  | 3,877.0 |  | 3,886.6
Regulated Operations [Member] | Residential [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | 40.7 | 39.5 |
Regulated Operations [Member] | Commercial [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | 36.6 | 38.2 |
Regulated Operations [Member] | Municipal [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | 14.0 | 18.2 |
Regulated Operations [Member] | Industrial [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | 114.9 | 121.7 |
Regulated Operations [Member] | Other Power Suppliers [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | 43.7 | 41.2 |
Regulated Operations [Member] | Other [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | 20.3 | 22.8 |
ALLETE Clean Energy [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue |  | 24.6 | 23.7 |
Net Income (Loss) |  | 8.1 | 6.7 |
Assets |  | 620.4 |  | 600.5
ALLETE Clean Energy [Member] | Long-term PSA [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | 18.6 | 17.8 |
ALLETE Clean Energy [Member] | Other [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue, Other Than Customer Revenue | [1] | 6.0 | 5.9 |
U.S. Water Services [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue |  | 38.2 | 32.1 |
Net Income (Loss) |  | (1.4) | (0.3) |
Assets |  | 287.4 |  | 292.4
U.S. Water Services [Member] | Point-in-Time [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | 22.3 | 21.8 |
U.S. Water Services [Member] | Contract [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | 9.5 | 8.9 |
U.S. Water Services [Member] | Capital Project [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | $ 6.4 | 1.4 |
Corporate and Other [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Number of Operating Segments | Segments |  | 2 |  |
Land in Minnesota (Acres) | a |  | 5,000 |  |
Operating Revenue |  | $ 25.2 | 28.2 |
Net Income (Loss) |  | 0.4 | (0.9) |
Assets |  | 287.3 |  | $ 300.5
Corporate and Other [Member] | Long-term Contract [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | 20.0 | 22.1 |
Corporate and Other [Member] | Other [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Operating Revenue | [1] | $ 5.2 | $ 6.1 |
[1] | With the adoption of new revenue recognition guidance, the Company has enhanced the presentation of business segment Operating Revenue. (See Note 1. Operations and Significant Accounting Policies.)

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