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Defined Benefit Plans - Components of Net Periodic Benefit Cost for Pension and Postretirement Benefits (Detail) - Net Periodic Benefit Cost - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 0.1 $ 0.2
Interest cost 8.2 7.6
Expected return on plan assets (8.8) (10.3)
Net periodic benefit income $ (0.5) $ (2.5)