Retirement Plans, Postretirement and Postemployment Benefits - Change in Projected Benefit Obligation (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension
Defined Benefit Plan Disclosure [Line Items]
Net projected benefit obligation at beginning of year	$ 977.8	$ 847.9
Service cost	39.2	30.8	$ 31.7
Interest cost	37.1	37.6	33.2
Actuarial loss	104.0	95.2
Plan amendments		6.4
Gross benefits paid	(46.2)	(40.1)
Net projected benefit obligation at end of year	1,111.9	977.8	847.9
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net projected benefit obligation at beginning of year	13.0	13.3
Service cost		0.1	0.1
Interest cost	0.4	0.6	0.5
Participants' contributions	0.7	1.2
Actuarial loss	0.5	1.0
Gross benefits paid	(2.0)	(3.2)
Net projected benefit obligation at end of year	$ 12.6	$ 13.0	$ 13.3