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, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 269.2	$ 286.9	$ 283.4
Projected benefit obligation	531.0	561.3	546.1
Funded status	(261.8)	(274.4)
Amounts reported within:
Accrued liabilities	(13.2)	(14.0)
Non-current liabilities	(248.6)	(260.4)
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	263.1	279.0
Projected benefit obligation	392.2	417.9
Funded status	(129.1)	(138.9)
Amounts reported within:
Accrued liabilities	(3.7)	(4.3)
Non-current liabilities	(125.4)	(134.6)
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6.1	7.9	9.5
Projected benefit obligation	138.8	143.4	$ 134.5
Funded status	(132.7)	(135.5)
Amounts reported within:
Accrued liabilities	(9.5)	(9.7)
Non-current liabilities	$ (123.2)	$ (125.8)