Commitments and Contingencies - Changes in Accrued Environmental Remediation and Related Costs (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Environmental Remediation Obligations [Abstract]
Balance at the beginning of the year	$ 99.7
Additions charged to expense	0.5
Payments, net	(1.5)
Other, net	(0.1)
Balance at the end of the period	98.6
Amounts recognized in our Condensed Consolidated Balance Sheet at the end of the period:
Current liabilities		$ 4.3	$ 4.5
Noncurrent liabilities		94.3	95.2
Total	$ 98.6	$ 98.6	$ 99.7