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Pension and Other Postretirement Benefit Plans - Plan Asset Allocations (Details) - shares
shares in Millions
Dec. 31, 2022
Dec. 31, 2021
Pension [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations 100.00% 100.00%
ALLETE Common Stock Included in Pension Plan Equity Securities (Shares) 0.0 0.0
Plan Asset Target Allocations 100.00%  
Pension [Member] | Equity Securities [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations 46.00% 49.00%
Plan Asset Target Allocations 49.00%  
Pension [Member] | Fixed Income [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations 50.00% 48.00%
Plan Asset Target Allocations 50.00%  
Pension [Member] | Private Equity [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations 0.00% 0.00%
Pension [Member] | Real Estate [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations 4.00% 3.00%
Plan Asset Target Allocations 1.00%  
Postretirement Health and Life [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations [1] 100.00% 100.00%
Plan Asset Target Allocations [2] 100.00%  
Postretirement Health and Life [Member] | Equity Securities [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations [1] 66.00% 70.00%
Plan Asset Target Allocations [2] 65.00%  
Postretirement Health and Life [Member] | Fixed Income [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations [1] 34.00% 29.00%
Plan Asset Target Allocations [2] 35.00%  
Postretirement Health and Life [Member] | Private Equity [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations [1] 0.00% 1.00%
Postretirement Health and Life [Member] | Real Estate [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations [1] 0.00% 0.00%
Plan Asset Target Allocations [2] 0.00%  
[1] Includes VEBAs and irrevocable grantor trusts.
[2] Includes VEBAs and irrevocable grantor trusts.