Employee Benefits - Change in Accumulated Postretirement Benefit Obligation (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Actuarial Loss (Gain)	$ 256.2	$ (131.4)
Other Postretirement Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning Balance	28.1	34.4
Service Cost	0.0	0.0	$ 0.1
Interest Cost	1.2	1.3	1.4
Actuarial Loss (Gain)	0.2	(6.7)
Net Claims Paid	(0.7)	(0.9)
Ending Balance	$ 28.8	$ 28.1	$ 34.4