Postretirement Health Care Plan Change in Accumulated in Postretirement Benefit Obligation (Detail) (Other Postretirement Plan, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Postretirement Plan
Schedule of Defined Benefit Plan Change in Accumulated Post Retirement Benefit Obligation [Line Items]
Beginning Balance	$ 31.2	$ 30.6
Service Cost	0.1	0.1	0.2
Interest Cost	1.5	1.2	1.3
Actuarial Loss	3.0	1.9
Net Claims Paid	(3.1)	(2.8)
Medicare Subsidy	0.7	0.2
Ending Balance	$ 33.4	$ 31.2	$ 30.6