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, 2012	Dec. 31, 2011
Pension Plans
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Net actuarial losses	$ (142.7)	$ (127.1)
Net prior service costs	(4.0)	(4.7)
Total recognized in AOCI- pretax	(146.7)	(131.8)
Other Retirement Plans
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Net actuarial losses	(6.0)	(6.1)
Total recognized in AOCI- pretax	$ (6.0)	$ (6.1)