Oil and Gas Reserves Information (Unaudited) - Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Reserves (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Cash Flows Relating to Proved Reserves:
Future Cash Flows	$ 3,891,953	$ 3,817,982
Future Costs:
Production	(1,260,580)	(1,307,923)
Development and Abandonment	(571,200)	(649,758)
Future Income Taxes	(192,600)	(451,708)
Future Net Cash Flows	1,867,573	1,408,593
10% Discount Factor	(776,913)	(601,376)
Standardized Measure of Discounted Future Net Cash Flows	$ 1,090,660	$ 807,217	$ 641,325	$ 887,798

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- Definition

Amount of estimated future accumulated cash inflow from the sale of proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for costs to be incurred in developing proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for income tax, based on the future income less future expense derived from production and sale of proved oil and gas reserves.

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Amount, after future accumulated cash outflow for development and production costs and future income tax, of estimated future accumulated cash inflow from the sale of proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for costs to be incurred in producing proved oil and gas reserves.

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Amount of discount on future accumulated cash inflow, after deducting future accumulated cash outflow for development and production costs and future income tax, from sale of proved oil and gas reserves, using a discount rate of ten percent a year.

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Amount of discounted future accumulated cash inflow relating to proved oil and gas reserves.

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