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Pension and Other Postretirement Benefit Plans - Plan Asset Allocations (Details) - shares
shares in Millions
Dec. 31, 2018
Dec. 31, 2017
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 32.00% 53.00%
Plan Asset Target Allocations 32.00%  
Pension [Member] | Fixed Income Securities [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations 60.00% 38.00%
Plan Asset Target Allocations 56.00%  
Pension [Member] | Private Equity [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations 5.00% 5.00%
Plan Asset Target Allocations 6.00%  
Pension [Member] | Real Estate [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations 3.00% 4.00%
Plan Asset Target Allocations 6.00%  
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] 62.00% 64.00%
Plan Asset Target Allocations [2] 60.00%  
Postretirement Health and Life [Member] | Fixed Income Securities [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations [1] 34.00% 31.00%
Plan Asset Target Allocations [2] 37.00%  
Postretirement Health and Life [Member] | Private Equity [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Actual Plan Asset Allocations [1] 4.00% 5.00%
Plan Asset Target Allocations [2] 3.00%  
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.