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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
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]      
Net periodic pension cost $ 3.3 $ 5.2 $ 2.7
Defined Benefit Pension Plan [Member]      
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]      
Interest cost 12.7 18.2 16.3
Actuarial loss (gain) amortization 15.7 12.7 14.2
Less: Expected return on plan assets (22.9) (23.6) (25.8)
Net periodic pension cost 5.5 7.3 4.7
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]      
Interest cost 0.3 0.5 0.5
Actuarial loss (gain) amortization (2.2) (2.3) (2.2)
Prior service credit amortization (0.3) (0.3) (0.3)
Net periodic pension cost $ (2.2) $ (2.1) $ (2.0)