Postretirement Benefit Plans - Net Loss and Prior Service Cost that will be Amortized from Accumulated Other Comprehensive Income Loss (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 13.1
Prior service cost (credit)	(10.0)
Total	3.1
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	8.7
Prior service cost (credit)	1.0
Total	9.7
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	4.4
Prior service cost (credit)	(11.0)
Total	$ (6.6)