Postretirement Benefit Plans - Summary of Funded Status of Postretirement Benefit Plans and Presentation of Such Balances within Consolidated Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.5	$ 321.8	$ 281.3
Projected benefit obligation	242.8	525.4	499.8
Funded status	(242.3)	(203.6)
Amounts reported within:
Non-current assets	0.2	24.5
Accrued liabilities	(15.0)	(14.2)
Non-current liabilities	(227.5)	(213.9)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.5	320.5
Projected benefit obligation	124.5	408.8
Funded status	(124.0)	(88.3)
Amounts reported within:
Non-current assets	0.2	24.5
Accrued liabilities	(5.8)	(4.9)
Non-current liabilities	(118.4)	(107.9)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	1.3	2.9
Projected benefit obligation	118.3	116.6	$ 118.6
Funded status	(118.3)	(115.3)
Amounts reported within:
Non-current assets	0.0	0.0
Accrued liabilities	(9.2)	(9.3)
Non-current liabilities	$ (109.1)	$ (106.0)