Derivative Financial Instruments - Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013 bbl	Dec. 31, 2012 bbl
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	673,000	464,000
Oil/Natural gas - liabilities	607,000	402,000
Oil/Natural gas - assets	$ 1,389	$ 2,188
Oil/Natural gas - liabilities	(965)	(1,590)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	10,600,000	120,000
Oil/Natural gas - liabilities	5,470,000	120,000
Oil/Natural gas - assets	3,435	19
Oil/Natural gas - liabilities	(829)	(24)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	387,000	402,000
Oil/Natural gas - liabilities	567,000	464,000
Oil/Natural gas - assets	595	1,636
Oil/Natural gas - liabilities	(1,309)	(2,139)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural gas - assets	5,796,000	120,000
Oil/Natural gas - liabilities	10,600,000	120,000
Oil/Natural gas - assets	1,236	24
Oil/Natural gas - liabilities	$ (3,330)	$ (19)