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, 2020	Dec. 31, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit (cost)	$ (0.2)	$ 0.1
Net actuarial gain (loss)	(15.6)	4.2
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service credit (cost)	0.1	0.6
Net actuarial gain (loss)	$ (0.4)	$ 1.4