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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 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]        
Recognition of actuarial losses (gains)     $ 0.3 $ (1.3)
Net Periodic Benefit Cost | Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 0.1 $ 0.1 0.4 0.4
Interest cost 7.7 8.3 23.0 25.0
Expected return on plan assets (10.3) (9.3) (30.8) (28.0)
Recognition of actuarial losses (gains) 0.3 0.3 0.3 0.3
Net periodic benefit income $ (2.2) (0.6) $ (7.1) (2.3)
Net Periodic Benefit Cost | Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Recognition of prior service credits       (5.1)
Recognition of actuarial losses (gains)   (1.6)   (1.6)
Net periodic benefit income   $ (1.6)   $ (6.7)