Employee Benefits - Postretirement Amounts Included in Accumulated Other Comprehensive Income (Detail) - Other Postretirement Plan - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net Actuarial Loss / (Gain)	$ 3.9	$ 0.3
Prior Service Cost	0.0	0.0
Gross Amount in Accumulated Other Comprehensive Income	3.9	0.3
Income Tax Effect	1.5	0.1
Net Amount in Accumulated Other Comprehensive Income	$ 2.4	$ 0.2