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, 2019	Dec. 31, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost/(credit)	$ 0.1	$ 0.5
Net actuarial loss	4.2	90.9
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service cost/(credit)	0.6
Net actuarial loss	$ 1.4	$ 1.9