Employee Benefits - Change in Accumulated Postretirement Benefit Obligation (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Actuarial (Gain)	$ (31.5)	$ 176.0
Other Postretirement Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning Balance	15.7	28.8
Service Cost	0.0	0.0	$ 0.0
Interest Cost	0.3	0.7	1.2
Plan Amendment	0.0	(12.7)
Actuarial (Gain)	(3.3)	(0.1)
Net Claims Paid	(2.5)	(1.0)
Ending Balance	$ 10.2	$ 15.7	$ 28.8