Defined Contribution and Defined Benefit Retirement Plans - Changes In Benefit Obligations Recognized In Accumulated Other Comprehensive Income (Loss) (Detail) - Defined Benefit Pension Plans - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Net actuarial losses	$ 36.8	$ 48.5	$ 24.3
Net actuarial losses	17.9	13.4	13.7
Prior service cost	0.2	0.2	0.2
Total	$ (18.7)	$ (34.9)	$ (10.4)