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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, 2018
Mar. 31, 2017
Pension Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 0.2 $ 0.1
Interest cost 7.6 8.4
Expected return on plan assets (10.3) (9.3)
Net periodic benefit income $ (2.5) (0.8)
Postretirement Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Recognition of prior service credits   (3.0)
Net periodic benefit income   $ (3.0)