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, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 281.3	$ 326.7	$ 269.2
Projected benefit obligation	499.8	557.1	531.0
Funded status	(218.5)	(230.4)
Amounts reported within:
Non-current assets	1.7	10.3
Accrued liabilities	(12.0)	(13.4)
Non-current liabilities	(208.2)	(227.3)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	278.4	321.5
Projected benefit obligation	381.2	419.0
Funded status	(102.8)	(97.5)
Amounts reported within:
Non-current assets	1.7	10.3
Accrued liabilities	(4.1)	(4.8)
Non-current liabilities	(100.4)	(103.0)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2.9	5.2	6.1
Projected benefit obligation	118.6	138.1	$ 138.8
Funded status	(115.7)	(132.9)
Amounts reported within:
Non-current assets	0.0	0.0
Accrued liabilities	(7.9)	(8.6)
Non-current liabilities	$ (107.8)	$ (124.3)