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Supplemental Oil and Gas Reserve Information - Standardized Measure of Discounted Future Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Future cash inflows	$ 11,321,992	$ 6,508,604	$ 2,620,530
Future production costs	(2,809,960)	(1,690,264)	(696,890)
Future development costs	(1,434,648)	(783,486)	(362,328)
Future income tax expense	(2,123,973)	(1,225,395)	(495,788)
Future net cash flows	4,953,411	2,809,459	1,065,524
10% annual discount for estimated timing of cash flows	(2,693,514)	(1,489,988)	(579,789)
Standardized measure of discounted future net cash flows	$ 2,259,897	$ 1,319,471	$ 485,735

This is the amount that is derived from using a discount rate of ten percent a year to reflect the timing of the future net cash flows relating to proved oil and gas reserves.

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- Definition

The estimated future income tax expenses that will be incurred based on the future income less future expenses derived from production and sale of proved oil and gas reserves in this geographic region.

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- Definition

These amounts are the result of subtracting future development and production costs and future income tax expenses from future cash inflows for each geographic region where proved oil and gas reserves exist.

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