Summary of Pre-Tax Net Actuarial Losses and Prior Service Cost Recognized in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Pension Plans [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Net actuarial losses	$ (144.6)	$ (165.5)
Net prior service costs	4.5	(2.7)
Total recognized in AOCI- pretax	(140.1)	(168.2)
Other Retirement Plans [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Net actuarial losses	(4.4)	$ (6.0)
Net prior service costs	1.2
Total recognized in AOCI- pretax	$ (3.2)	$ (6.0)