Employee Benefits - Change in Accumulated Postreitrement Benefit Obligation (Detail) - Other Postretirement Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning Balance	$ 34.1	$ 32.2
Service Cost	0.1	0.1	$ 0.1
Interest Cost	1.4	1.5	1.4
Actuarial Loss / (Gain)	(0.2)	2.7
Net Claims Paid	(1.0)	(2.4)
Medicare Subsidy	0.0	0.0
Ending Balance	$ 34.4	$ 34.1	$ 32.2