Derivative Financial Instruments (Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014 bbl	Dec. 31, 2013 bbl
Financial Institution Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount	470,000	356,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 14,357	$ 1,004
Oil/Natural Gas Derivative Liabilities Notional Amount	197,000	1,574,000
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	(1,670)	(2,704)
Financial Institution Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount	12,235,000	14,240,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	12,707	2,903
Oil/Natural Gas Derivative Liabilities Notional Amount	16,755,000	22,510,000
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	(4,095)	(3,212)
Customer Counterparties [Member] | Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount	197,000	1,574,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	1,670	2,818
Oil/Natural Gas Derivative Liabilities Notional Amount	470,000	356,000
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	(14,318)	(991)
Customer Counterparties [Member] | Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets Notional Amount	16,755,000	22,850,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	4,095	3,301
Oil/Natural Gas Derivative Liabilities Notional Amount	12,235,000	13,900,000
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (12,646)	$ (2,805)