Employee Benefits - Postretirement Health Care Plan Status (Detail) - Other Postretirement Plan - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Net Postretirement Benefit Obligation	$ 10.2	$ 15.7	$ 28.8
Retirees and Dependents
Defined Benefit Plan Disclosure [Line Items]
Net Postretirement Benefit Obligation	8.7	13.2
Actives Eligible for Benefits
Defined Benefit Plan Disclosure [Line Items]
Net Postretirement Benefit Obligation	$ 1.5	$ 2.5