Commitments and Contingencies - Changes in Accrued Environmental Remediation and Related Costs (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Balance at the beginning of the period	$ 111,909
Additions charged to expense, net	4,291
Payments, net	(739)
Balance at the end of the period	115,461
Amounts recognized in the Condensed Consolidated Balance Sheet at the end of the period:
Current liability		$ 18,253	$ 18,253	$ 5,302
Noncurrent liability		$ 97,208	97,208	106,607
Balance at the end of the period	$ 111,909		$ 115,461	$ 111,909