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Derivative Financial Instruments - Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015 bbl	Dec. 31, 2014 bbl
Financial Institution Counterparties [Member] \| Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount	724,000	470,000
Oil/Natural Gas Derivative Liabilities, Notional Amount	538,000	197,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	$ 15,137	$ 14,357
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	(2,121)	(1,670)
Financial Institution Counterparties [Member] \| Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount	9,385,000	12,235,000
Oil/Natural Gas Derivative Liabilities, Notional Amount	11,985,000	16,755,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	11,489	12,707
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	(3,591)	(4,095)
Customer Counterparties [Member] \| Oil Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount	547,000	197,000
Oil/Natural Gas Derivative Liabilities, Notional Amount	715,000	470,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	2,124	1,670
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	(15,100)	(14,318)
Customer Counterparties [Member] \| Natural Gas Commodity Derivative [Member]
Derivative Instruments [Line Items]
Oil/Natural Gas Derivative Assets, Notional Amount	11,985,000	16,755,000
Oil/Natural Gas Derivative Liabilities, Notional Amount	9,385,000	12,235,000
Oil/Natural Gas Derivative Assets, Estimated Fair Value	3,591	4,095
Oil/Natural Gas Derivative Liabilities, Estimated Fair Value	$ (11,435)	$ (12,646)