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Operations and Significant Accounting Policies (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Mar. 31, 2020
Dec. 31, 2019
Cash, Cash Equivalents and Restricted Cash [Abstract]        
Cash and Cash Equivalents $ 159.0 $ 44.3 $ 67.0 $ 69.3
Restricted Cash included in Prepayments and Other 7.1 0.8 6.1 2.8
Restricted Cash, Non-Current 10.9 20.1 5.8 20.4
Total Cash, Cash Equivalents and Restricted Cash 177.0 65.2 78.9 92.5
Inventories – Net [Abstract]        
Fuel [1] 20.4 23.1    
Materials and Supplies 53.6 51.1    
Total Inventories – Net 74.0 74.2    
Other Non-Current Assets [Abstract]        
Contract Assets [2] 24.8 25.5    
Operating Lease, Right-of-Use Asset 20.2 22.4    
ALLETE Properties 18.2 18.2    
Restricted Cash, Non-Current 10.9 20.1 $ 5.8 $ 20.4
Other Postretirement Benefit Plans 34.7 34.2    
Other 82.5 86.4    
Total Other Non-Current Assets 191.3 206.8    
Other Current Liabilites [Abstract]        
PSAs 12.5 12.5    
Public Utilities, Fuel Adjustment Clause 1.8 3.7    
Operating Lease Liabilities 5.7 5.9    
Other 58.9 44.6    
Total Other Current Liabilities 78.9 66.7    
Other Non-Current Liabilities [Abstract]        
Asset Retirement Obligation (a) [3] 170.8 166.6    
PSAs 48.9 52.1    
Operating Lease Liabilities 14.5 16.5    
Other 46.0 50.1    
Total Other Non-Current Liabilities 280.2 285.3    
BNI Energy        
Business Segments [Line Items]        
Other Receivables $ 25.0 $ 25.0    
[1] Fuel consists primarily of coal inventory at Minnesota Power.
[2] Contract Assets consist of 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.
[3] The asset retirement obligation is primarily related to our Regulated Operations and is funded through customer rates over the life of the related assets. Additionally, BNI Energy funds its obligation through its cost-plus coal supply agreements for which BNI Energy has recorded a receivable of $25.0 million in Other Non-Current Assets on the Consolidated Balance Sheet as of March 31, 2021 and December 31, 2020.