Retirement Plans, Postretirement and Postemployment Benefits - Schedule of Change In Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Balance at beginning of year	$ 868.0
Employer contributions	88.4	$ 58.9
Balance at end of year	1,037.9	868.0
Pension
Defined Benefit Plan Disclosure [Line Items]
Balance at beginning of year	868.0	717.9
Actual return on plan assets, net	127.7	131.3
Employer contributions	88.4	58.9
Gross benefits paid	(46.2)	(40.1)
Balance at end of year	1,037.9	868.0
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	1.3	2.0
Participants' contributions	0.7	1.2
Gross benefits paid	$ (2.0)	$ (3.2)