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Operations and Significant Accounting Policies - Narrative (Details) | 3 Months Ended | 12 Months Ended
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Sep. 30, 2024 USD ($) | Dec. 31, 2024 USD ($) customer energy_market entity unit state segment MW | Dec. 31, 2023 USD ($) MW | Dec. 31, 2022 USD ($)
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of reportable segments | segment |  | 2 |  |
Prepayments and other | $ |  | $ 99,700,000 | $ 83,600,000 |
Impairment of long-lived assets held-for-use | $ |  |  | 0 | $ 0
Power received | $ | $ 14,900,000 |  |  |
Government Assistance, Asset, Decrease, Noncurrent, Statement of Financial Position [Extensible Enumeration] | Regulatory Assets |  |  |
Residual value of leased asset | $ |  | 3,000,000 |  |
Contract asset - noncurrent | $ |  | 15,900,000 | 18,500,000 |
Capitalized contract cost, amortization | $ |  | $ 2,400,000 | 2,400,000 |
Large Power Customers | Customer Concentration Risk |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of customers | customer |  | 8 |  |
Receivables from large power customers | $ |  | $ 13,300,000 | $ 11,200,000 |
Number of entities | entity |  | 2 |  |
Large Power Customers | Customer Concentration Risk | Largest Customer | Consolidated Operating Revenue |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Concentration risk |  | 27.00% | 21.00% | 26.00%
Boswell Unit 3 | 2021 Integrated Resource Plan | MPUC |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Net book value | $ |  | $ 220,000,000 |  |
Boswell Unit 4 | 2021 Integrated Resource Plan | MPUC |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Net book value | $ |  | $ 395,000,000 |  |
Glen Ullin Energy Center |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Generating capacity subject to tax equity financing | MW |  | 106 | 106 |
South Peak |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Generating capacity subject to tax equity financing | MW |  | 80 | 80 |
Diamond Spring |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Generating capacity subject to tax equity financing | MW |  | 303 | 303 |
Caddo |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Generating capacity subject to tax equity financing | MW |  | 303 | 303 |
New Energy Solar Energy Facilities |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Generating capacity subject to tax equity financing | MW |  | 14 |  |
Square Butte PPA | Square Butte Coal-fired Unit |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Output being purchased (MW) | MW |  | 227.5 |  |
Square Butte | Square Butte PPA | Square Butte Coal-fired Unit |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Generating capacity counterparty owned (MW) | MW |  | 455 |  |
New Energy | Prepayments And Other |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Prepayments and other | $ |  | $ 32,400,000 | $ 21,500,000 |
Natural Gas-Fired | Nemadji Trail Energy Center |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Generating capacity jointly owned (MW) | MW |  | 600 |  |
Nobles 2 |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Ownership percentage |  | 49.00% |  |
Generating capacity subject to tax equity financing | MW |  | 250 | 250 |
Minnesota Power | Square Butte | Square Butte PPA | Square Butte Coal-fired Unit |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Expected output entitlement |  | 50.00% |  |
Minnesota Power | Wind Turbine Generators | Tenaska | Nobles 2 PPA |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Generating capacity counterparty owned (MW) | MW |  | 250 |  |
Contract term |  | 20 years |  |
New Energy |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Discount rate - input for goodwill analysis |  | 12.50% |  |
Terminal growth rate - input for goodwill analysis |  | 3.50% |  |
Regulated Operations | Alternative Programs |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Revenue collection period following the annual period it is recognized |  | 24 months |  |
Regulated Operations | Alternative Programs | Minimum |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Revenue recognition payment term |  | 15 days |  |
Regulated Operations | Alternative Programs | Maximum |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Revenue recognition payment term |  | 30 days |  |
Regulated Operations | Municipal Customers | Wholesale Electric Service | Long-term Contract with Customer | Municipal |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of customers | customer |  | 14 |  |
Length of notice required to terminate contract |  | 3 years |  |
Regulated Operations | Industrial Customers | Long-term Contract with Customer | Industrial | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01 |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Revenue, remaining performance obligation, expected timing of satisfaction, period |  | 3 years |  |
Revenue, remaining performance obligation, amount | $ |  | $ 60,000,000 |  |
Regulated Operations | Industrial Customers | Long-term Contract with Customer | Industrial | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01 |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Revenue, remaining performance obligation, expected timing of satisfaction, period |  | 1 year |  |
Regulated Operations | Industrial Customers | Long-term Contract with Customer | Industrial | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01 |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Revenue, remaining performance obligation, expected timing of satisfaction, period |  |  |  |
Revenue, remaining performance obligation, amount | $ |  | $ 10,000,000 |  |
Regulated Operations | Industrial Customers | Retail Electric Service | Industrial |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Percentage of total regulated utility k wh sales |  | 56.00% |  |
Regulated Operations | Industrial Customers | Retail Electric Service | Long-term Contract with Customer | Industrial |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Length of notice required to terminate contract |  | 4 years |  |
Large power customer contracts | customer |  | 8 |  |
Contract serving requirement | MW |  | 10 |  |
Regulated Operations | Other Power Supplier Customers | Sale of Energy under PSA | Long-term Contract with Customer | Other Power Suppliers |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of customers | customer |  | 1 |  |
Regulated Operations | Large Power Customers | Customer Concentration Risk | Largest Customer | Consolidated Operating Revenue |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Concentration risk |  | 33.00% | 32.00% | 32.00%
ALLETE Clean Energy |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Energy generation owned, operated or developed (MW) | MW |  | 1,600 |  |
Number of major energy markets | energy_market |  | 5 |  |
Number of states operated in | state |  | 8 |  |
Corporate and Other | BNI Energy |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of customers | unit |  | 2 |  |
Corporate and Other | BNI Energy | Square Butte Coal-fired Unit |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of customers | unit |  | 1 |  |
Electric Rates | Regulated Operations | Minnesota Power | Municipal Customers |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of customers | customer |  | 14 |  |
Electric Rates | Retail Customers | Regulated Operations | Minnesota Power |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of customers | customer |  | 150,000 |  |
Electric Rates | Retail Customers | Regulated Operations | SWL&P |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of customers | customer |  | 15,000 |  |
Natural Gas | Retail Customers | Regulated Operations | SWL&P |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of customers | customer |  | 13,000 |  |
Water | Retail Customers | Regulated Operations | SWL&P |  |  |  |
Other Income (Expense) - Other [Line Items] |  |  |  |
Number of customers | customer |  | 10,000 |  |

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