Retirement Plans and Other Post-Retirement Benefits - Information for Pension Plans with Accumulated Benefit Obligation in Excess of Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Pension Plans With Accumulated Benefit Obligations In Excess Of Plan Assets [Abstract]
Projected benefit obligation	$ 47.3	$ 45.7
Accumulated benefit obligation	$ 47.3	$ 45.7