Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	$ 1,624	$ 1,946
Derivative liabilities, Estimated Fair Value	(1,138)	(989)
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	1,624	1,946
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities, Estimated Fair Value	$ (1,138)	(989)
Oil [Member]
Derivatives, Fair Value [Line Items]
Notional Units	Barrels
Derivative assets, Notional Amount	$ 88	86
Derivative assets, Estimated Fair Value	356	604
Derivative liabilities, Notional Amount	(88)	(86)
Derivative liabilities, Estimated Fair Value	$ (269)	(378)
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Notional Units	MMBTUs
Derivative assets, Notional Amount	$ 3,930	3,920
Derivative assets, Estimated Fair Value	1,268	1,342
Derivative liabilities, Notional Amount	(3,930)	(3,920)
Derivative liabilities, Estimated Fair Value	$ (869)	$ (611)