Commitments, Contingencies and Legal Proceedings - Schedule of Reserves (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Environmental Remediation Obligations [Abstract]
Beginning balance	$ 43,144	$ 52,788
Payments	(3,054)	(9,644)
Assumption of WTM I escrow	4,746
Accretion	165
Ending balance	$ 45,001	$ 43,144