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, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost/(credit)	$ 14.8	$ 12.0
Net actuarial loss	165.9	185.0
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost/(credit)	(0.6)	(0.8)
Net actuarial loss	$ (7.4)	$ (4.8)