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Retirement Plans - Narrative (Detail) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]        
Pension and postretirement contributions   $ 8.6 $ 17.9 $ 16.4
Defined benefit plan, accumulated benefit obligation $ 921.9 $ 1,049.9 921.9  
Threshold for recognition in net periodic benefit costs, percentage of projected benefit obligation or fair value of plan assets   10.00%    
Multi-Employer Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Total contributions   $ 6.5 6.6  
MEPP withdrawal obligations related to facility closures        
Defined Benefit Plan Disclosure [Line Items]        
Other Charges   $ 2.9 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   $ 9.0    
Pension and postretirement expected contributions for next year   9.2    
Medicare reimbursements   0.0 0.0  
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Medicare reimbursements   6.6 $ 6.4  
Other Postretirement Benefit Plans, Defined Benefit | Canada        
Defined Benefit Plan Disclosure [Line Items]        
OPEB retiree contribution term 3 years      
OPEB expiration year 2021      
Adjustments in OPEB liability $ 31.1      
EGWP        
Defined Benefit Plan Disclosure [Line Items]        
Medicare reimbursements   $ 6.6