Employee Benefits - Postretirement Health Care Plan Status (Detail) - Other Postretirement Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Accumulated Postretirement Benefit Obligation	$ 34.4	$ 34.1	$ 32.2
Retirees and Dependents
Defined Benefit Plan Disclosure [Line Items]
Accumulated Postretirement Benefit Obligation	27.7	26.4
Actives Eligible for Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated Postretirement Benefit Obligation	$ 6.7	$ 7.7