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Employee Benefits - Postretirement Health Care Plan Status (Detail) - Other Postretirement Plan - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Net Postretirement Benefit Obligation $ 15.7 $ 28.8 $ 28.1
Retirees and Dependents      
Defined Benefit Plan Disclosure [Line Items]      
Net Postretirement Benefit Obligation 13.2 25.2  
Actives Eligible for Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Net Postretirement Benefit Obligation $ 2.5 $ 3.6