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Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]        
Cash and Cash Equivalents $ 45.1 $ 44.3 $ 69.3  
Restricted Cash included in Prepayments and Other 0.3 0.8 2.8  
Restricted Cash included in Other Non-Current Assets 2.3 20.1 20.4  
Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows 47.7 65.2 92.5 $ 79.0
Accounts Receivable [Abstract]        
Billed 100.8 93.5    
Unbilled 24.7 20.9    
Less: Allowance for Doubtful Accounts 1.8 2.5    
Total Accounts Receivable 123.7 111.9    
Inventories – Net [Abstract]        
Fuel [1] 18.7 23.1    
Materials and Supplies 56.1 51.1    
Construction of Wind Energy Facilities [2] 22.9 0.0    
Total Inventories – Net 97.7 74.2    
Property, Plant and Equipment [Line Items]        
Impairment of Long-Lived Assets Held-for-use 0.0 0.0 0.0  
Other Non-Current Assets [Abstract]        
Contract Asset - Noncurrent [3] 23.3 25.5    
Operating Lease, Right-of-Use Asset 16.4 22.4    
ALLETE Properties 19.4 18.2    
Restricted Cash, Noncurrent 2.3 20.1 $ 20.4  
Other Postretirement Benefit Plans 64.8 34.2    
Other 87.5 86.4    
Total Other Non-Current Assets 213.7 206.8    
Customer Deposits [4] 27.2 7.4    
PSAs 12.6 12.5    
Manufactured Gas Plant - Current [5] 12.8 0.0    
Fuel Adjustment Clause 5.0 3.7    
Operating Lease Liabilities, Current 4.8      
Redeemable Non-Controlling Interest [6] 30.6 0.0    
Other 40.0 37.2    
Total Other Current Liabilities 133.0 66.7    
Asset Retirement Obligation 184.5 166.6    
PSAs 39.5 52.1    
Operating Lease Liabilities, Non-Current 11.6      
Other 45.2 50.1    
Total Other Non-Current Liabilities $ 280.8 $ 285.3    
Income Taxes [Abstract]        
More-Likely-Than-Not Percentage 50.00% 50.00%    
Taconite Harbor [Member] | Resource Package [Member] | MPUC [Member]        
Property, Plant and Equipment [Line Items]        
Net Book Value $ 45.0      
Boswell Unit 3 [Member] | 2021 Integrated Resource Plan [Member] | MPUC [Member]        
Property, Plant and Equipment [Line Items]        
Net Book Value 245.0      
Boswell Unit 4 [Member] | 2021 Integrated Resource Plan [Member] | MPUC [Member]        
Property, Plant and Equipment [Line Items]        
Net Book Value 435.0      
Other Current Liabilities        
Operating Lease Liabilities, Current 4.8 $ 5.9    
Other Noncurrent Liabilities        
Operating Lease Liabilities, Non-Current $ 11.6 $ 16.5    
[1] Fuel consists primarily of coal inventory at Minnesota Power.
[2] Project costs related to ALLETE Clean Energy’s Northern Wind and Red Barn wind projects which are expected be sold in late 2022 and 2023, respectively. (See Other Current Liabilities.)
[3] Contract Assets include payments made to customers as an incentive to execute or extend service agreements. The contract payments are being amortized over the term of the respective agreements as a reduction to revenue.
[4] Primarily related to deposits received by ALLETE Clean Energy for the Northern Wind and Red Barn wind projects which are expected be sold in late 2022 and 2023 respectively. (See Inventories – Net.)
[5] The manufactured gas plant represents the current liability for remediation of a former manufactured gas plant site located in Superior, Wisconsin, and formerly operated by SWL&P. (See Note 8. Commitments, Guarantees and Contingencies.)
[6] Amount reclassified from Non-Controlling Interest in Subsidiaries resulting from the exercise of an option to buy out a non-controlling interest, which was paid in the first quarter of 2022.