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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, 2020
Mar. 31, 2019
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Net periodic cost (benefit gain) $ 0.8 $ 1.3
Defined Benefit Pension Plan [Member]    
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Interest cost 3.2 4.5
Actuarial loss (gain) amortization 3.9 3.2
Less: Expected return on plan assets (5.7) (5.9)
Net periodic cost (benefit gain) 1.4 1.8
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) $ (0.6) $ (0.5)