Postretirement Health Care Plan Change in Accumulated in Postretirement Benefit Obligation (Detail) - Other Postretirement Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning Balance	$ 32.2	$ 33.4
Service Cost	0.1	0.1	$ 0.1
Interest Cost	1.5	1.4	1.5
Actuarial Loss / (Gain)	2.7	(0.2)
Net Claims Paid	(2.4)	(2.5)
Medicare Subsidy	0.0	0.0
Ending Balance	$ 34.1	$ 32.2	$ 33.4