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Defined Benefit Plans - Amounts in Accumulated Other Comprehensive loss that have not yet been Recognized as Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Recognition of actuarial loss $ (3.2) $ (34.1) $ (3.8)
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Recognition of actuarial loss (70.6) (121.6)  
Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Recognition of actuarial loss (0.2) (1.0)  
Other Accumulated Other Comprehensive Income | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Beginning Balance 87.7 71.8  
Recognition of actuarial loss (2.7) (34.1)  
Current year actuarial loss 2.1 50.1  
Net actuarial loss due to curtailment (0.6) (0.1)  
Ending Balance 86.5 87.7 71.8
Other Accumulated Other Comprehensive Income | Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Beginning Balance (0.3) (0.3)  
Recognition of actuarial loss (0.1) (0.6)  
Current year actuarial loss 1.0 0.6  
Ending Balance $ 0.6 $ (0.3) $ (0.3)