Commitments and Contingencies - Changes in Accrued Environmental Remediation and Related Costs (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Environmental Remediation Obligations [Abstract]
Balance at the beginning of the year	$ 117.5
Additions charged to expense, net	2.9
Payments, net	(3.7)
Balance at the end of period	116.7
Amounts recognized in our Condensed Consolidated Balance Sheet at the end of the period:
Current liabilities		$ 19.0	$ 6.8
Noncurrent liabilities		97.7	110.7
Total	$ 117.5	$ 116.7	$ 117.5