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Postretirement Health Care and Life Insurance Benefits - Reconciliation of Postretirement Plans' Funded Status to the Balance Sheet (Details) - Postretirement Health Care and Life Insurance Benefits - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Change in benefit obligation:      
Projected benefit obligation at beginning of period $ 189.9 $ 258.7  
Service cost 0.5 0.8 $ 1.0
Interest cost 10.2 7.0 10.5
Benefits Paid and Administrative Fees (20.9) (21.2)  
Actuarial gain (1.7) (55.4)  
Projected benefit obligation at end of period 178.0 189.9 258.7
Change in plan assets:      
Fair value of plan assets at beginning of period 17.4 26.1  
Actual return on plan assets 1.5 (3.4)  
Employer contributions 16.3 15.9  
Benefits paid and administrative fees (net of Medicare Part D reimbursements) (20.9) (21.2)  
Fair value of plan assets at end of period 14.3 17.4 $ 26.1
Funded status at end of period (163.7) (172.5)  
Less: Current portion (included in “Accounts payable and accrued expenses”) 15.3 16.0  
Noncurrent obligation (included in “Other noncurrent liabilities”) $ (148.4) $ (156.5)