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Defined Benefit Plans - Components of Net Periodic Benefit Cost for Pension and Postretirement Benefits (Detail) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]        
Recognition of actuarial losses (gains)   $ 3.8 $ (0.5) $ 1.9
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost   0.5 0.6  
Interest cost   30.7 33.3  
Recognition of actuarial losses (gains)   (63.1) 40.6  
Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Recognition of actuarial losses (gains) $ 0.9 (0.2) (1.4)  
Net Periodic Benefit Cost | Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost   0.5 0.6 9.6
Interest cost   30.7 33.3 34.4
Expected return on plan assets   (41.0) (37.3) (37.2)
Recognition of actuarial losses (gains)   3.9 0.9  
Net periodic benefit (income) cost   (5.9) (2.5) 6.8
Net Periodic Benefit Cost | Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost       0.3
Recognition of actuarial losses (gains)   (0.1) (1.4) 1.9
Amortization of prior service credits     (5.1) (13.5)
Net periodic benefit (income) cost   $ (0.1) $ (6.5) $ (11.3)