Commitments and Contingencies - Changes in Accrued Environmental Remediation and Related Costs (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Environmental Remediation Obligations [Abstract]
Balance at the beginning of the year	$ 117.5
Additions charged to expense, net	4.3
Payments, net	(0.5)
Balance at the end of period	121.3
Amounts recognized in our Condensed Consolidated Balance Sheet at the end of the period:
Current liabilities		$ 19.7	$ 6.8
Noncurrent liabilities		101.6	110.7
Total	$ 117.5	$ 121.3	$ 117.5