Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	$ 1,017	$ 1,946
Derivative liabilities, Estimated Fair Value	(665)	(989)
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	1,017	1,946
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities, Estimated Fair Value	$ (665)	(989)
Oil [Member]
Derivatives, Fair Value [Line Items]
Notional Units	Barrels
Derivative assets, Notional Amount	$ 61	86
Derivative assets, Estimated Fair Value	190	604
Derivative liabilities, Notional Amount	(61)	(86)
Derivative liabilities, Estimated Fair Value	$ (131)	(378)
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Notional Units	MMBTUs
Derivative assets, Notional Amount	$ 2,540	3,920
Derivative assets, Estimated Fair Value	827	1,342
Derivative liabilities, Notional Amount	(2,540)	(3,920)
Derivative liabilities, Estimated Fair Value	$ (534)	$ (611)