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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
Mar. 31, 2017
Mar. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Recognition of actuarial losses   $ 0.9
Net Periodic Benefit Cost | Pension Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 0.1 2.8
Interest cost 8.4 8.7
Expected return on plan assets (9.3) (9.5)
Net periodic benefit (income) expense (0.8) 2.0
Net Periodic Benefit Cost | Postretirement Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Interest cost   0.1
Recognition of prior service credits (3.0) (2.3)
Recognition of actuarial losses   0.9
Net periodic benefit (income) expense $ (3.0) $ (1.3)