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Operations and Significant Accounting Policies - Narrative (Details)
 3 Months Ended 12 Months Ended
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