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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Recognition of actuarial losses (gains) $ 3.2 $ 34.1 $ 3.8
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.4 0.4  
Interest cost 28.3 32.9  
Recognition of actuarial losses (gains) 70.6 121.6  
Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.4 0.2  
Interest cost 0.2 0.2  
Recognition of actuarial losses (gains) 0.2 1.0  
Net Periodic Benefit Cost | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.4 0.4 0.5
Interest cost 28.3 32.9 30.7
Expected return on plan assets (32.8) (35.2) (41.0)
Recognition of actuarial losses (gains) 2.7 34.1 3.9
Settlement/Curtailment losses (gains) 0.6 0.1  
Net periodic benefit cost (income) (0.8) 32.3 (5.9)
Net Periodic Benefit Cost | Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 0.4 0.2  
Interest cost 0.2 0.2  
Recognition of actuarial losses (gains) 0.1 0.6 (0.1)
Settlement/Curtailment losses (gains)   (0.1)  
Amortization of prior service credits   0.2  
Net periodic benefit cost (income) $ 0.7 $ 1.1 $ (0.1)