Derivative Financial Instruments - Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013 bbl	Dec. 31, 2012 bbl	Mar. 31, 2012 bbl
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	574,000	464,000	1,415,000
Oil/Natural gas - liabilities	1,016,000	402,000	1,716,000
Oil/Natural gas - assets	$ 1,426	$ 2,188	$ 3,928
Oil/Natural gas - liabilities	(2,816)	(1,590)	(11,286)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	3,050,000	120,000	1,290,000
Oil/Natural gas - liabilities	9,035,000	120,000	300,000
Oil/Natural gas - assets	733	19	2,862
Oil/Natural gas - liabilities	(1,614)	(24)	(40)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	1,016,000	402,000	1,716,000
Oil/Natural gas - liabilities	574,000	464,000	1,415,000
Oil/Natural gas - assets	2,909	1,636	11,384
Oil/Natural gas - liabilities	(1,369)	(2,139)	(3,907)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	9,995,000	120,000	300,000
Oil/Natural gas - liabilities	2,090,000	120,000	1,290,000
Oil/Natural gas - assets	1,691	24	40
Oil/Natural gas - liabilities	$ (728)	$ (19)	$ (2,830)