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Employee Benefit Plans - Components of Net Periodic Cost for Defined Benefit Pension Plan and for Postretirement Benefit Plan (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Net periodic cost (benefit gain) $ 1.3 $ 0.6
Defined Benefit Pension Plan [Member]    
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Interest cost 4.5 4.1
Actuarial loss (gain) amortization 3.2 3.6
Less: Expected return on plan assets (5.9) (6.5)
Net periodic cost (benefit gain) 1.8 1.2
Other Postretirement Benefits Plan [Member]    
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Interest cost 0.1 0.1
Actuarial loss (gain) amortization (0.6) (0.6)
Prior service credit amortization   (0.1)
Net periodic cost (benefit gain) $ (0.5) $ (0.6)