Derivative Financial Instruments - Notional Amounts and Estimated Fair Values of Outstanding Derivative Positions (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	$ 5,300	$ 3,072
Derivative liabilities, Estimated Fair Value	(3,775)	(1,667)
Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets, Estimated Fair Value	5,300
Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities, Estimated Fair Value	(3,775)
Oil [Member]
Derivatives, Fair Value [Line Items]
Notional Units	Barrels
Derivative assets, Notional Amount	275
Derivative assets, Estimated Fair Value	3,825
Derivative liabilities, Notional Amount	(275)
Derivative liabilities, Estimated Fair Value	(3,382)
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Notional Units	MMBTUs
Derivative assets, Notional Amount	5,610
Derivative assets, Estimated Fair Value	1,475
Derivative liabilities, Notional Amount	(5,610)
Derivative liabilities, Estimated Fair Value	$ (393)