Summary of Pre-Tax Net Actuarial Losses and Prior Service Cost Recognized in Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Pension Plans
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Net actuarial losses	$ (165.5)	$ (84.6)
Net prior service costs	(2.7)	(3.3)
Total recognized in AOCI- pretax	(168.2)	(87.9)
Other Retirement Plans
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Net actuarial losses	(6.0)	(2.4)
Net prior service costs
Total recognized in AOCI- pretax	$ (6.0)	$ (2.4)