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Defined Benefit Plans - Components of Net Periodic Benefit Cost for Pension and Postretirement Benefits (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Recognition of actuarial losses (gains) $ 34.1 $ 3.8 $ (0.5)
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.4 0.5  
Interest cost 32.9 30.7  
Recognition of actuarial losses (gains) 121.6 (63.1)  
Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.2    
Interest cost 0.2    
Recognition of actuarial losses (gains) 1.0 (0.2)  
Net Periodic Benefit Cost | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.4 0.5 0.6
Interest cost 32.9 30.7 33.3
Expected return on plan assets (35.2) (41.0) (37.3)
Recognition of actuarial losses (gains) 34.1 3.9 0.9
Settlement/Curtailment losses (gains) 0.1    
Net periodic benefit cost (income) 32.3 (5.9) (2.5)
Net Periodic Benefit Cost | Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.2    
Interest cost 0.2    
Recognition of actuarial losses (gains) 0.6 (0.1) (1.4)
Settlement/Curtailment losses (gains) (0.1)    
Amortization of prior service credits 0.2   (5.1)
Net periodic benefit cost (income) $ 1.1 $ (0.1) $ (6.5)