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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Recognition of actuarial (loss) gain $ (34.1) $ (3.8) $ 0.5
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Recognition of actuarial (loss) gain (121.6) 63.1  
Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Recognition of actuarial (loss) gain (1.0) 0.2  
Other Accumulated Other Comprehensive Income | Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Beginning Balance 71.8 67.2  
Recognition of actuarial (loss) gain (34.1) (3.9)  
Current year actuarial loss (gain) 50.1 8.5  
Net actuarial loss due to curtailment (0.1)    
Ending Balance 87.7 71.8 $ 67.2
Other Accumulated Other Comprehensive Income | Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Beginning Balance (0.3)    
Recognition of actuarial (loss) gain (0.6) 0.1  
Current year actuarial loss (gain) 0.6 (0.4)  
Ending Balance $ (0.3) $ (0.3)