Commodity Derivative Activities - Summary of Gains and Losses on Derivatives (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative [Line Items]
Total loss on commodity derivatives not designated as hedges	$ (981)	$ (260)
Commodity Contract [Member]
Derivative [Line Items]
Gain (loss) on commodity derivatives not designated as hedges, settled	(384)	142
Loss on commodity derivatives not designated as hedges, not settled	(597)	(402)
Total loss on commodity derivatives not designated as hedges	$ (981)	$ (260)