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Operations and Significant Accounting Policies - Business Segments (Details) | 12 Months Ended
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Dec. 31, 2018 a Customers MW | Dec. 31, 2017
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 |
Minnesota Power [Member] | Square Butte [Member] | Square Butte [Member] | Square Butte PPA [Member] |  |
Business Segments [Line Items] |  |
Minnesota Power Output Entitlement (Percent) | 50.00% |
Regulated Operations [Member] | Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member] |  |
Business Segments [Line Items] |  |
Number of Customers | 145,000 |
Regulated Operations [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Electric Rates [Member] |  |
Business Segments [Line Items] |  |
Number of Customers | 16 |
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Electric Rates [Member] |  |
Business Segments [Line Items] |  |
Number of Customers | 15,000 |
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Natural Gas [Member] |  |
Business Segments [Line Items] |  |
Number of Customers | 13,000 |
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Water [Member] |  |
Business Segments [Line Items] |  |
Number of Customers | 10,000 |
ALLETE Clean Energy [Member] | ALLETE Clean Energy [Member] |  |
Business Segments [Line Items] |  |
Generating Capacity Owned (MW) | MW | 545 |
Corporate and Other [Member] |  |
Business Segments [Line Items] |  |
Land in Minnesota (Acres) | a | 4,000 |
Corporate and Other [Member] | BNI Energy [Domain] |  |
Business Segments [Line Items] |  |
Number of Customers | 2 |
Corporate and Other [Member] | BNI Energy [Domain] | Square Butte [Member] |  |
Business Segments [Line Items] |  |
Number of Customers | 1 |

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