Retirement Plans, Postretirement and Postemployment Benefits - Net Periodic Benefit Credit or Cost Not Yet Recognized (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit) - Gross	$ 5.8	$ 6.4
Actuarial (gain) loss - Gross	245.9	229.4
Total - Gross	251.7	235.8
Prior service cost (credit) - Net of tax	3.7	4.0
Actuarial (gain) loss - Net of tax	157.1	144.5
Total - Net of tax	160.8	148.5
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service cost (credit) - Gross	(2.2)	(3.0)
Actuarial (gain) loss - Gross	(2.0)	(2.7)
Total - Gross	(4.2)	(5.7)
Prior service cost (credit) - Net of tax	(1.4)	(1.9)
Actuarial (gain) loss - Net of tax	(1.3)	(1.7)
Total - Net of tax	$ (2.7)	$ (3.6)