XML 109 R84.htm IDEA: XBRL DOCUMENT v3.24.0.1
Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($)
 12 Months Ended
Jan. 12, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]        
ESOP compensation expense   $ 13,700,000 $ 12,000,000 $ 11,500,000
Subsequent Event        
Defined Benefit Plan Disclosure [Line Items]        
Employer contribution $ 25,000,000      
Pension        
Defined Benefit Plan Disclosure [Line Items]        
Employer contributions to defined benefit pension plans - cash   17,300,000 0 10,300,000
Employer contribution   26,200,000 12,300,000  
Expected employer contributions in 2024   2,000,000    
Net loss   256,900,000 260,200,000  
Prior Service Credit   $ 0 $ 100,000  
ALLETE common stock included in pension plan equity securities (in shares)   0 0  
Postretirement Health and Life        
Defined Benefit Plan Disclosure [Line Items]        
Employer contribution     $ 300,000  
Expected employer contributions in 2024   $ 0    
Net loss   (24,800,000) (9,200,000)  
Prior Service Credit   (33,800,000) (13,200,000)  
Postretirement Health and Life | VEBA        
Defined Benefit Plan Disclosure [Line Items]        
Employer contributions to other postretirement benefit plans   0 0 0
Postretirement Health and Life | Irrevocable Grantor Trust        
Defined Benefit Plan Disclosure [Line Items]        
Employer contributions to other postretirement benefit plans   0 0 $ 0
Long-term investments   $ 12,800,000 $ 11,800,000