Postretirement Benefit Plans - Amount Recognized in Accumulated Other Comprehensive Income Loss (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 232.1	$ 166.3
Prior service cost (benefit)	(67.6)	(19.4)
Total	164.5	146.9
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	169.1	126.9
Prior service cost (benefit)	6.6	2.8
Total	175.7	129.7
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	63.0	39.4
Prior service cost (benefit)	(74.2)	(22.2)
Total	$ (11.2)	$ 17.2