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, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 321.8	$ 281.3	$ 326.7
Projected benefit obligation	525.4	499.8	557.1
Funded status	(203.6)	(218.5)
Amounts reported within:
Non-current assets	24.5	1.7
Accrued liabilities	(14.2)	(12.0)
Non-current liabilities	(213.9)	(208.2)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	320.5	278.4
Projected benefit obligation	408.8	381.2
Funded status	(88.3)	(102.8)
Amounts reported within:
Non-current assets	24.5	1.7
Accrued liabilities	(4.9)	(4.1)
Non-current liabilities	(107.9)	(100.4)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1.3	2.9	5.2
Projected benefit obligation	116.6	118.6	$ 138.1
Funded status	(115.3)	(115.7)
Amounts reported within:
Non-current assets	0.0	0.0
Accrued liabilities	(9.3)	(7.9)
Non-current liabilities	$ (106.0)	$ (107.8)