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Operations and Significant Accounting Policies - Business Segments (Details) $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Oct. 01, 2021 USD ($) | Dec. 31, 2021 USD ($) a Customers MW | Dec. 31, 2021 USD ($) a Customers MW | Dec. 31, 2020
Business Segments [Line Items] |  |  |  |
Number of Reportable Segments |  |  | 3 | 3
Square Butte [Member] | Square Butte PPA [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Minnesota Power Output Entitlement (MW) | MW |  |  | 227.5 |
Nemadji Trail Energy Center [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Estimated Capital Expenditures, Including Past Expenditures | $ |  | $ 700.0 | $ 700.0 |
Square Butte [Member] | Square Butte [Member] | Square Butte PPA [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Generating Capacity Counterparty Owned (MW) | MW |  | 455 | 455 |
Minnesota Power [Member] | Square Butte [Member] | Square Butte [Member] | Square Butte PPA [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Expected Output Entitlement (Percent) |  |  | 50.00% |
Minnesota Power Output Entitlement (Percent) |  |  | 50.00% |
South Shore Energy | Nemadji Trail Energy Center [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Estimated Capital Expenditures, Including Past Expenditures | $ |  | $ 150.0 | $ 150.0 |
Capital Cost Spent to Date | $ |  | $ 5.0 | $ 5.0 |
Regulated Operations [Member] | Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Number of Customers |  | 150,000 | 150,000 |
Regulated Operations [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Electric Rates [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Number of Customers |  | 15 | 15 |
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Electric Rates [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Number of Customers |  | 15,000 | 15,000 |
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Natural Gas [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Number of Customers |  | 13,000 | 13,000 |
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Water [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Number of Customers |  | 10,000 | 10,000 |
ALLETE Clean Energy [Member] | ALLETE Clean Energy [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Energy Generation Owned (MW) | MW |  | 1,300 | 1,300 |
Generating Capacity Under Construction (MW) | MW |  | 200 | 200 |
U.S. Water Services [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Approximate Proceeds Expected or Received from Divestiture of Business | $ |  |  | $ 270.0 |
Corporate and Other [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Land in Minnesota (Acres) | a |  | 4,000 | 4,000 |
Corporate and Other [Member] | South Shore Energy | Nemadji Trail Energy Center [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Gain on Sale After Tax | $ | $ 8.5 |  | $ 8.5 |
Approximate Sales Price | $ | $ 20.0 |  |  |
Corporate and Other [Member] | BNI Energy [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Number of Customers |  | 2 | 2 |
Corporate and Other [Member] | BNI Energy [Member] | Square Butte [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Number of Customers |  | 1 | 1 |
Nobles 2 [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Ownership Percentage |  | 49.00% | 49.00% |
Natural Gas-Fired [Member] | Nemadji Trail Energy Center [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Generating Capacity Jointly Owned (MW) | MW |  | 600 | 600 |
Natural Gas-Fired [Member] | Basin Electric Power Cooperative [Member] | Nemadji Trail Energy Center [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Expected Output Entitlement (Percent) | 30.00% |  |  |
Minnesota Power Output Entitlement (Percent) | 30.00% |  |  |
Natural Gas-Fired [Member] | Dairyland Power Cooperative [Member] | Nemadji Trail Energy Center [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Expected Output Entitlement (Percent) | 50.00% |  |  |
Minnesota Power Output Entitlement (Percent) | 50.00% |  |  |
Natural Gas-Fired [Member] | South Shore Energy | Nemadji Trail Energy Center [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Expected Output Entitlement (Percent) | 20.00% | 20.00% |  |
Minnesota Power Output Entitlement (Percent) | 20.00% | 20.00% |  |
Wind Turbine Generators [Member] | Minnesota Power [Member] | Tenaska [Member] | Nobles 2 PPA [Member] |  |  |  |
Business Segments [Line Items] |  |  |  |
Generating Capacity Counterparty Owned (MW) | MW |  | 250 | 250 |
Contract Term (Years) |  |  | 20 years |

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