Derivative Financial Instruments (Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013 bbl	Dec. 31, 2012 bbl
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount	356,000	464,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 1,004	$ 2,188
Oil/Natural Gas Derivative Liabilities Notional Amount	1,574,000	402,000
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	(2,704)	(1,590)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount	14,240,000	120,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	2,903	19
Oil/Natural Gas Derivative Liabilities Notional Amount	22,510,000	120,000
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	(3,212)	(24)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount	1,574,000	402,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	2,818	1,636
Oil/Natural Gas Derivative Liabilities Notional Amount	356,000	464,000
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	(991)	(2,139)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount	22,850,000	120,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	3,301	24
Oil/Natural Gas Derivative Liabilities Notional Amount	13,900,000	120,000
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (2,805)	$ (19)