Employee Benefits - Change in Accumulated Postretirement Benefit Obligation (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Actuarial Loss (Gain)	$ 176.0	$ 256.2
Other Postretirement Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning Balance	28.8	28.1
Service Cost	0.0	0.0	$ 0.0
Interest Cost	0.7	1.2	1.3
Plan Amendment	(12.7)	0.0
Actuarial Loss (Gain)	(0.1)	0.2
Net Claims Paid	(1.0)	(0.7)
Ending Balance	$ 15.7	$ 28.8	$ 28.1