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Employee Benefit Plans - Components of Net Periodic Cost for Defined Benefit Pension Plan and Other Postretirement Benefit Plan (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Net periodic pension cost (postretirement benefit gain)     $ 1.2 $ 0.6
Defined Benefit Pension Plan [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Interest cost $ 4.1 $ 4.3 8.2 8.6
Actuarial loss (gain) amortization 3.6 3.2 7.2 6.4
Less: Expected return on plan assets (6.5) (6.6) (13.0) (13.2)
Net periodic pension cost (postretirement benefit gain) 1.2 0.9 2.4 1.8
Other Postretirement Benefits Plan [Member]        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service benefit (0.1) (0.1) (0.2) (0.2)
Interest cost 0.1 0.1 0.2 0.2
Actuarial loss (gain) amortization (0.6) (0.6) (1.2) (1.2)
Net periodic pension cost (postretirement benefit gain) $ (0.6) $ (0.6) $ (1.2) $ (1.2)