Commitments and Contingencies - Changes in Accrued Environmental Remediation and Related Costs (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Environmental Remediation Obligations [Abstract]
Balance at the beginning of the year	$ 98.6
Payments, net	(0.4)
Balance at the end of the period	98.2
Amounts recognized in our Condensed Consolidated Balance Sheet at the end of the period:
Current liabilities		$ 3.5	$ 3.4
Noncurrent liabilities		94.7	95.2
Total	$ 98.2	$ 98.2	$ 98.6