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Pension and Other Postretirement Benefits - Pension Plan Assets Fair Value (Details) - Pensions - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets $ 1,041.3 $ 1,044.1 $ 1,372.0
Quoted Prices in Active Markets for Identical Assets (Level 1)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 220.7 259.5  
Significant Other Observable Inputs (Level 2)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 820.6 784.6  
Significant Unobservable Inputs (Level 3)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 0.0 0.0  
Cash and short-term investments      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 3.0 22.8  
Cash and short-term investments | Quoted Prices in Active Markets for Identical Assets (Level 1)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 3.0 22.8  
Cash and short-term investments | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 0.0 0.0  
Cash and short-term investments | Significant Unobservable Inputs (Level 3)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 0.0 0.0  
Investment contracts      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 114.9 116.4  
Investment contracts | Quoted Prices in Active Markets for Identical Assets (Level 1)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 0.0 0.0  
Investment contracts | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 114.9 116.4  
Investment contracts | Significant Unobservable Inputs (Level 3)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 0.0 0.0  
U.S. Government Securities      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 204.6 207.4  
U.S. Government Securities | Quoted Prices in Active Markets for Identical Assets (Level 1)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 204.6 207.4  
U.S. Government Securities | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 0.0 0.0  
U.S. Government Securities | Significant Unobservable Inputs (Level 3)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 0.0 0.0  
Mutual funds      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 13.1 29.3  
Mutual funds | Quoted Prices in Active Markets for Identical Assets (Level 1)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 13.1 29.3  
Mutual funds | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 0.0 0.0  
Mutual funds | Significant Unobservable Inputs (Level 3)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 0.0 0.0  
Corporate debt instruments      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 705.7 668.2  
Corporate debt instruments | Quoted Prices in Active Markets for Identical Assets (Level 1)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 0.0 0.0  
Corporate debt instruments | Significant Other Observable Inputs (Level 2)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets 705.7 668.2  
Corporate debt instruments | Significant Unobservable Inputs (Level 3)      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair value of plan assets $ 0.0 $ 0.0