Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	$ 256	$ 1,569
Derivative liabilities, Estimated Fair Value	(116)	(1,306)
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	256	1,569
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities, Estimated Fair Value	$ (116)	(1,306)
Oil [Member]
Derivatives, Fair Value [Line Items]
Notional Units	Barrels
Derivative assets, Notional Amount	$ 46	83
Derivative assets, Estimated Fair Value	108	576
Derivative liabilities, Notional Amount	(46)	(83)
Derivative liabilities, Estimated Fair Value	$ (59)	(496)
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Notional Units	MMBTUs
Derivative assets, Notional Amount	$ 830	1,900
Derivative assets, Estimated Fair Value	148	993
Derivative liabilities, Notional Amount	(830)	(1,900)
Derivative liabilities, Estimated Fair Value	$ (57)	$ (810)