Employee Benefits - Change in Accumulated Postreitrement Benefit Obligation (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Actuarial Loss / (Gain)	$ (131.4)	$ 164.5
Other Postretirement Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning Balance	34.4	34.1
Service Cost	0.0	0.1	$ 0.1
Interest Cost	1.3	1.4	1.5
Actuarial Loss / (Gain)	(6.7)	(0.2)
Net Claims Paid	(6.3)	(1.0)
Medicare Subsidy	5.4	0.0
Ending Balance	$ 28.1	$ 34.4	$ 34.1