Retirement Plans and Other Post-Retirement Benefits - Components of Amounts Recognized as Accumulated Other Comprehensive Income on Pre-Tax Basis (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost/(credit)	$ 16.1	$ 14.8
Net actuarial loss	145.6	165.9
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost/(credit)	(0.5)	(0.6)
Net actuarial loss	$ (5.6)	$ (7.4)