Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	$ 528	$ 1,569
Derivative liabilities, Estimated Fair Value	(329)	(1,306)
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	528	1,569
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities, Estimated Fair Value	$ (329)	(1,306)
Oil [Member]
Derivatives, Fair Value [Line Items]
Notional Units	Barrels
Derivative assets, Notional Amount	$ 63	83
Derivative assets, Estimated Fair Value	161	576
Derivative liabilities, Notional Amount	(63)	(83)
Derivative liabilities, Estimated Fair Value	$ (101)	(496)
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Notional Units	MMBTUs
Derivative assets, Notional Amount	$ 1,770	1,900
Derivative assets, Estimated Fair Value	367	993
Derivative liabilities, Notional Amount	(1,770)	(1,900)
Derivative liabilities, Estimated Fair Value	$ (228)	$ (810)