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Postretirement Health Care and Life Insurance Benefits - Fair Value of Plan Assets in Non-Represented Trust by Asset (Details) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Other Postretirement Benefits Plan        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets $ 14.3 $ 17.4 $ 26.1  
Other Postretirement Benefits Plan | U.S. equity securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 3.0 3.7    
Other Postretirement Benefits Plan | International equity securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.9 1.1    
Other Postretirement Benefits Plan | Corporate bonds        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 5.2 7.1    
Other Postretirement Benefits Plan | U.S. government securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 4.6 5.0    
Other Postretirement Benefits Plan | Cash funds        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.6 0.5    
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 6.3 7.3    
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan | U.S. equity securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 3.0 3.7    
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan | International equity securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.9 1.1    
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan | Corporate bonds        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.0 0.0    
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan | U.S. government securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 1.8 2.0    
Fair Value, Inputs, Level 1 | Other Postretirement Benefits Plan | Cash funds        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.6 0.5    
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 8.0 10.1    
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan | U.S. equity securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.0 0.0    
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan | International equity securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.0 0.0    
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan | Corporate bonds        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 5.2 7.1    
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan | U.S. government securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 2.8 3.0    
Fair Value, Inputs, Level 2 | Other Postretirement Benefits Plan | Cash funds        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.0 0.0    
Fair Value, Inputs, Level 3        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.0 430.4 $ 0.3 $ 1.2
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.0 0.0    
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan | U.S. equity securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.0 0.0    
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan | International equity securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.0 0.0    
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan | Corporate bonds        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.0 0.0    
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan | U.S. government securities        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets 0.0 0.0    
Fair Value, Inputs, Level 3 | Other Postretirement Benefits Plan | Cash funds        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined benefit plan, assets $ 0.0 $ 0.0