Defined Contribution and Defined Benefit Retirement - Schedule of Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss) (Detail) - Defined Benefit Pension Plans - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Net actuarial gain (loss)	$ (37.7)	$ (47.2)	$ (27.0)
Amortization of unrecognized prior service cost (credit)	0.2	0.2	0.2
Amortization of unrecognized net actuarial loss	19.4	15.0	15.2
Total	$ (18.1)	$ (32.0)	$ (11.6)