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Operations and Significant Accounting Policies - Business Segments (Details)
$ in Millions
12 Months Ended
Dec. 31, 2020
USD ($)
a
Customers
MW
Dec. 31, 2019
Feb. 04, 2021
USD ($)
MW
Business Segments [Line Items]      
Number of Reportable Segments 3 3  
Subsequent Event [Member]      
Business Segments [Line Items]      
Approximate Proceeds from Sale of Assets | $     $ 210
Anticipated Generating Capacity to be Sold (MW) | MW     120
Chanarambie and Viking Wind Energy Facilities [Member] | Subsequent Event [Member]      
Business Segments [Line Items]      
Energy Generation Owned (MW) | MW     100
Square Butte [Member] | Square Butte PPA [Member]      
Business Segments [Line Items]      
Minnesota Power Output Entitlement (MW) | MW 227.5    
Square Butte [Member] | Square Butte [Member] | Square Butte PPA [Member]      
Business Segments [Line Items]      
Generating Capacity Counterparty Owned (MW) | MW 455    
Minnesota Power [Member] | Square Butte [Member] | Square Butte [Member] | Square Butte PPA [Member]      
Business Segments [Line Items]      
Minnesota Power Output Entitlement (Percent) 50.00%    
U.S. Water Services [Member]      
Business Segments [Line Items]      
Approximate Proceeds Expected or Received from Divestiture of Business | $ $ 270    
Regulated Operations [Member] | Minnesota Power [Member] | Retail Customers [Member] | Electric Rates [Member]      
Business Segments [Line Items]      
Number of Customers | Customers 145,000    
Regulated Operations [Member] | Minnesota Power [Member] | Municipal Customers [Member] | Electric Rates [Member]      
Business Segments [Line Items]      
Number of Customers | Customers 15    
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Electric Rates [Member]      
Business Segments [Line Items]      
Number of Customers | Customers 15,000    
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Natural Gas [Member]      
Business Segments [Line Items]      
Number of Customers | Customers 13,000    
Regulated Operations [Member] | SWL&P [Member] | Retail Customers [Member] | Water [Member]      
Business Segments [Line Items]      
Number of Customers | Customers 10,000    
ALLETE Clean Energy [Member] | ALLETE Clean Energy [Member]      
Business Segments [Line Items]      
Energy Generation Owned (MW) | MW 1,000    
Generating Capacity Under Construction (MW) | MW 300    
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 | Customers 2    
Corporate and Other [Member] | BNI Energy [Domain] | Square Butte [Member]      
Business Segments [Line Items]      
Number of Customers | Customers 1    
Nobles 2 [Member]      
Business Segments [Line Items]      
Ownership Percentage 49.00%    
Wind Turbine Generators [Member] | Resource Package [Member] | MPUC [Member] | Minnesota Power [Member] | Tenaska [Member] | Nobles 2 PPA [Member]      
Business Segments [Line Items]      
Generating Capacity Counterparty Owned (MW) | MW 250    
Contract Term (Years) 20 years