Commitments, Contingencies and Legal Proceedings - Schedule of Reserves (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Environmental Remediation Obligations [Abstract]
Beginning balance	$ 52,788	$ 17,105
Payments	(9,644)	(4,317)
Accruals		40,000
Ending balance	$ 43,144	$ 52,788