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Business Segments (Details) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 USD ($) a | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) a Segments | Jun. 30, 2017 USD ($) | Dec. 31, 2017 USD ($)
Business Segments [Line Items] |  |  |  |  |  |
Number of Reportable Segments | Segments |  |  |  | 3 |  |
Operating Revenue |  | $ 344.1 | $ 353.3 | $ 702.3 | $ 718.9 |
Net Income (Loss) |  | 31.3 | 36.9 | 82.3 | 85.9 |
Assets |  | 5,114.7 |  | $ 5,114.7 |  | $ 5,080.0
Regulated Operations [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Number of Operating Segments | Segments |  |  |  | 3 |  |
Operating Revenue |  | 257.8 | 264.9 | $ 528.0 | 546.5 |
Net Income (Loss) |  | 26.0 | 32.4 | 69.9 | 75.9 |
Assets |  | 3,898.2 |  | 3,898.2 |  | 3,886.6
Regulated Operations [Member] | Residential [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | 30.7 | 27.6 | 71.4 | 67.1 |
Regulated Operations [Member] | Commercial [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | 36.2 | 33.7 | 72.8 | 71.9 |
Regulated Operations [Member] | Municipal [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | 13.7 | 12.3 | 27.7 | 30.5 |
Regulated Operations [Member] | Industrial [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | 115.3 | 121.6 | 230.2 | 243.3 |
Regulated Operations [Member] | Other Power Suppliers [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | 42.7 | 41.7 | 86.4 | 82.9 |
Regulated Operations [Member] | Conservation Improvement Programs [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1],[2] | 0.0 | 5.5 | 0.0 | 5.5 |
Regulated Operations [Member] | Other [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | 19.2 | 22.5 | 39.5 | 45.3 |
ALLETE Clean Energy [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue |  | 18.3 | 19.6 | 42.9 | 43.3 |
Net Income (Loss) |  | 6.8 | 3.8 | 14.9 | 10.5 |
Assets |  | 620.2 |  | 620.2 |  | 600.5
ALLETE Clean Energy [Member] | Long-term PSA [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | 12.4 | 13.7 | 31.0 | 31.5 |
ALLETE Clean Energy [Member] | Other [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue, Other Than Customer Revenue | [1] | 5.9 | 5.9 | 11.9 | 11.8 |
U.S. Water Services [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue |  | 41.5 | 38.4 | 79.7 | 70.5 |
Net Income (Loss) |  | 0.2 | 0.6 | (1.2) | 0.3 |
Assets |  | 291.8 |  | 291.8 |  | 292.4
U.S. Water Services [Member] | Point-in-Time [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | 25.7 | 23.9 | 48.0 | 45.7 |
U.S. Water Services [Member] | Contract [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | 9.5 | 8.8 | 19.0 | 17.7 |
U.S. Water Services [Member] | Capital Project [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | $ 6.3 | 5.7 | $ 12.7 | 7.1 |
Corporate and Other [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Number of Operating Segments | Segments |  |  |  | 2 |  |
Land in Minnesota (Acres) | a |  | 4,000 |  | 4,000 |  |
Operating Revenue |  | $ 26.5 | 30.4 | $ 51.7 | 58.6 |
Net Income (Loss) |  | (1.7) | 0.1 | (1.3) | (0.8) |
Assets |  | 304.5 |  | 304.5 |  | $ 300.5
Corporate and Other [Member] | Long-term Contract [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | 22.7 | 23.2 | 42.7 | 45.3 |
Corporate and Other [Member] | Other [Member] |  |  |  |  |  |
Business Segments [Line Items] |  |  |  |  |  |
Operating Revenue | [1] | $ 3.8 | $ 7.2 | $ 9.0 | $ 13.3 |
[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.)
[2] | See Note 6. Regulatory Matters.

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