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, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 326.7	$ 269.2	$ 286.9
Projected benefit obligation	557.1	531.0	561.3
Funded status	(230.4)	(261.8)
Pension and Other Postretirement Defined Benefit Plans, Assets, Noncurrent	10.3	0.0
Amounts reported within:
Accrued liabilities	(13.4)	(13.2)
Non-current liabilities	(227.3)	(248.6)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	321.5	263.1
Projected benefit obligation	419.0	392.2
Funded status	(97.5)	(129.1)
Pension and Other Postretirement Defined Benefit Plans, Assets, Noncurrent	10.3	0.0
Amounts reported within:
Accrued liabilities	(4.8)	(3.7)
Non-current liabilities	(103.0)	(125.4)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5.2	6.1	7.9
Projected benefit obligation	138.1	138.8	$ 143.4
Funded status	(132.9)	(132.7)
Pension and Other Postretirement Defined Benefit Plans, Assets, Noncurrent	0.0	0.0
Amounts reported within:
Accrued liabilities	(8.6)	(9.5)
Non-current liabilities	$ (124.3)	$ (123.2)