Benefit Plans - Amounts Recorded in Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Non-current liabilities	$ (102.0)	$ (96.4)
Accumulated benefit obligation	119.3	107.1
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Current liabilities	(5.6)	(5.3)
Non-current liabilities	(81.5)	(75.8)
Benefit obligations in excess of the fair value of plan assets	(87.1)	(81.1)
Other post-retirement plans
Defined Benefit Plan Disclosure [Line Items]
Current liabilities	(1.5)	(1.7)
Non-current liabilities	(12.3)	(12.9)
Benefit obligations in excess of the fair value of plan assets	$ (13.8)	$ (14.6)