Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	$ 2,357	$ 6,124
Derivative liabilities, Estimated Fair Value	(1,335)	(4,756)
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	2,357	6,124
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities, Estimated Fair Value	$ (1,335)	(4,756)
Oil [Member]
Derivatives, Fair Value [Line Items]
Notional Units	Barrels
Derivative assets, Notional Amount	$ 55	312
Derivative assets, Estimated Fair Value	1,197	4,629
Derivative liabilities, Notional Amount	(55)	(312)
Derivative liabilities, Estimated Fair Value	$ (971)	(4,271)
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Notional Units	MMBTUs
Derivative assets, Notional Amount	$ 3,790	2,010
Derivative assets, Estimated Fair Value	1,160	1,495
Derivative liabilities, Notional Amount	(3,790)	(2,010)
Derivative liabilities, Estimated Fair Value	$ (364)	$ (485)