Summary of Pre-Tax Net Actuarial Losses and Prior Service Cost Recognized in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Pension Plans [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Net actuarial losses	$ (104.0)	$ (133.9)
Net prior service costs	4.5	4.5
Total recognized in AOCI- pretax	(99.5)	(129.4)
Other Retirement Plans [Member]
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Net actuarial losses	(3.0)	(4.0)
Net prior service costs	0.6	0.9
Total recognized in AOCI- pretax	$ (2.4)	$ (3.1)