Employee Benefits - Postretirement Health Care Plan Status (Detail) - Other Postretirement Plan - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net Postretirement Benefit Obligation	$ 28.1	$ 34.4	$ 34.1
Retirees and Dependents
Defined Benefit Plan Disclosure [Line Items]
Net Postretirement Benefit Obligation	23.5	27.7
Actives Eligible for Benefits
Defined Benefit Plan Disclosure [Line Items]
Net Postretirement Benefit Obligation	$ 4.6	$ 6.7