Retirement Plans - Amounts Recognized on Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Accrued benefit cost (included in accrued liabilities)	$ (351.2)	$ (311.3)
Pension liabilities	(89.5)	(113.6)
OPEB plan liabilities	(55.8)	(61.7)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension cost (included in other noncurrent assets)	44.2	41.2
Accrued benefit cost (included in accrued liabilities)	(2.5)	(2.7)
Pension liabilities	(89.5)	(113.6)
OPEB plan liabilities	0.0	0.0
Net liabilities recognized in the Consolidated Balance Sheets	(47.8)	(75.1)
OPEB
Defined Benefit Plan Disclosure [Line Items]
Prepaid pension cost (included in other noncurrent assets)	0.0	0.0
Accrued benefit cost (included in accrued liabilities)	(0.4)	(0.4)
Pension liabilities	0.0	0.0
OPEB plan liabilities	(55.8)	(61.7)
Net liabilities recognized in the Consolidated Balance Sheets	$ (56.2)	$ (62.1)