Derivative Financial Instruments - Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 bbl	Dec. 31, 2012 bbl
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	588,000	464,000
Oil/Natural gas - liabilities	1,250,000	402,000
Oil/Natural gas - assets	$ 378	$ 2,188
Oil/Natural gas - liabilities	(3,959)	(1,590)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	9,035,000	120,000
Oil/Natural gas - liabilities	5,130,000	120,000
Oil/Natural gas - assets	3,404	19
Oil/Natural gas - liabilities	(730)	(24)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	1,621,000	402,000
Oil/Natural gas - liabilities	217,000	464,000
Oil/Natural gas - assets	4,106	1,636
Oil/Natural gas - liabilities	(354)	(2,139)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	5,130,000	120,000
Oil/Natural gas - liabilities	9,035,000	120,000
Oil/Natural gas - assets	730	24
Oil/Natural gas - liabilities	$ (3,315)	$ (19)