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Defined Benefit Plans - Obligations and Funded Status (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Change in the Projected Benefit Obligation (PBO):      
Recognition of actuarial losses (gains) $ 34.1 $ 3.8 $ (0.5)
Change in Plan Assets:      
Beginning balance 599.6    
Ending balance 677.2 599.6  
Pension Benefits [Member]      
Change in the Projected Benefit Obligation (PBO):      
Projected benefit obligation at beginning of year 763.2 832.4  
Service cost 0.4 0.5  
Interest cost 32.9 30.7  
Recognition of actuarial losses (gains) 121.6 (63.1)  
Benefits paid (41.0) (37.3)  
Projected benefit obligation at end of year 877.1 763.2 832.4
Accumulated benefit obligation at end of year (excludes the impact of future compensation increases) 877.1 763.2  
Change in Plan Assets:      
Beginning balance 599.6 656.6  
Actual return on plan assets 106.8 (30.7)  
Employer contributions 11.8 11.0  
Benefits paid (41.0) (37.3)  
Ending balance 677.2 599.6 656.6
Funded status (Fair value of plan assets less PBO) (199.9) (163.6)  
Postretirement Benefits [Member]      
Change in the Projected Benefit Obligation (PBO):      
Projected benefit obligation at beginning of year 1.4 1.6  
Service cost 0.2    
Interest cost 0.2    
Plan amendments 1.6    
Recognition of actuarial losses (gains) 1.0 (0.2)  
Benefits paid (0.7)    
Curtailment gain (0.1)    
Projected benefit obligation at end of year 3.6 1.4 $ 1.6
Change in Plan Assets:      
Employer contributions 0.7    
Benefits paid (0.7)    
Funded status (Fair value of plan assets less PBO) $ (3.6) $ (1.4)