Employee Benefits - Postretirement Health Care Plan Status (Detail) - Other Postretirement Plan - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Net Postretirement Benefit Obligation	$ 28.8	$ 28.1	$ 34.4
Retirees and Dependents
Defined Benefit Plan Disclosure [Line Items]
Net Postretirement Benefit Obligation	25.2	23.5
Actives Eligible for Benefits
Defined Benefit Plan Disclosure [Line Items]
Net Postretirement Benefit Obligation	$ 3.6	$ 4.6