Commitments and Contingencies - Changes in Accrued Environmental Remediation and Related Costs (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Balance at the beginning of the period	$ 111,909
Additions charged to expense, net	2,671
Payments, net	(3,098)
Balance at the end of the period	111,482
Amounts recognized in the Condensed Consolidated Balance Sheet at the end of the period:
Current liability		$ 17,762	$ 5,302
Noncurrent liability		93,720	106,607
Balance at the end of the period	$ 111,909	$ 111,482	$ 111,909