Retirement Plans and Other Post-Retirement Benefits - Summary of Amounts Recognized in Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net amount recognized	$ (47.3)	$ 7.7
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net amount recognized	(6.0)	(6.5)
Cash and Cash Equivalents [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net amount recognized		53.4
Current Liabilities [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net amount recognized	(2.3)	(2.3)
Current Liabilities [Member] | Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net amount recognized	(1.2)	(1.4)
Other Long-Term Liabilities [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net amount recognized	(45.0)	(43.4)
Other Long-Term Liabilities [Member] | Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net amount recognized	$ (4.8)	$ (5.1)