Basis of Presentation - Inventory - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Inventory [Line Items]
Inventory and other	$ 20,146	$ 26,573
Materials and supplies inventory
Inventory [Line Items]
Inventory and other	10,200	9,400
Commodity inventory
Inventory [Line Items]
Inventory and other	$ 2,400	$ 8,300