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Retirement Plans - Narrative (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Pension and postretirement contributions $ 9.5 $ 8.6 $ 17.9
Defined benefit plan, accumulated benefit obligation $ 1,126.2 1,049.9  
Threshold for recognition in net periodic benefit costs, percentage of projected benefit obligation or fair value of plan assets 10.00%    
MEPP withdrawal obligations related to facility closures      
Defined Benefit Plan Disclosure [Line Items]      
Other Charges $ 40.0 2.9 $ 2.9
Hedging Investments      
Defined Benefit Plan Disclosure [Line Items]      
Target asset allocation percentage 50.00%    
Return Seeking Securities      
Defined Benefit Plan Disclosure [Line Items]      
Target asset allocation percentage 50.00%    
Pension Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Pension and postretirement contributions $ 9.2    
Pension and postretirement expected contributions for next year 5.5    
Medicare reimbursements 0.0 0.0  
Total contributions to MEPPs 10.3 $ 6.5  
EGWP      
Defined Benefit Plan Disclosure [Line Items]      
Medicare reimbursements $ 7.9