Employee Benefits - Net Periodic Postretirement (Benefit) Expense (Detail) - Other Postretirement Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 0.1	$ 0.1	$ 0.1
Interest Cost	1.4	1.5	1.4
Expected Return on Plan Assets	0.0	0.0	0.0
Amortization
Net Gain	0.0	0.0	0.0
Prior Service Benefit	0.0	0.0	0.0
Net Periodic Pension Expense	$ 1.5	$ 1.6	$ 1.5