Basis of Presentation - Inventory - Additional Information (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Inventory [Line Items]
Inventory and other	$ 21,967,000	$ 21,346,000
Materials and supplies inventory
Inventory [Line Items]
Inventory and other	8,400,000	12,100,000
Commodity inventory
Inventory [Line Items]
Inventory and other	$ 364,000	$ 508,000