Employee Benefits - Net Periodic Postretirement (Benefit) Expense (Detail) - Other Postretirement Plan - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Aug. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service Cost		$ 0.0	$ 0.0	$ 0.0
Interest Cost		0.7	1.2	1.3
Expected Return on Plan Assets		0.0	0.0	0.0
Amortization
Net Gain		(0.6)	(1.1)	0.0
Prior Service Benefit	$ (12.7)	(0.3)	0.0	0.0
Net Periodic Pension Expense		$ (0.2)	$ 0.1	$ 1.3