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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, 2021
Mar. 31, 2020
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Net periodic cost (benefit gain) $ 2.2 $ 0.8
Defined Benefit Pension Plan [Member]    
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Interest cost 1.7 3.2
Actuarial loss (gain) amortization 4.3 3.9
Less: Expected return on plan assets (3.3) (5.7)
Net periodic cost (benefit gain) 2.7 1.4
Other Postretirement Benefits Plan [Member]    
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]    
Interest cost   0.1
Actuarial loss (gain) amortization (0.5) (0.6)
Prior service credit amortization   (0.1)
Net periodic cost (benefit gain) $ (0.5) $ (0.6)