v2.4.0.6

Disclosures About Oil and Gas Producing Activities - Summary of Changes in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Natural Gas Reserves (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Standardized measure of discounted future net cash flows, Beginning balance	$ 414,381	$ 204,232	$ 79,991
Net change in sales and transfer prices and in production (lifting) costs related to future production	147,421	334,104	120,520
Changes in estimated future development costs	(486,435)	(395,037)	(65,718)
Sales and transfers of oil and gas produced during the period	(100,634)	(89,253)	(46,031)
Net change due to extensions, discoveries and improved recovery	467,822	291,501	30,240
Net change due to purchase of minerals in place		119,780	15,696
Net change due to revisions in quantity estimates	(210,296)	(84,988)	80,564
Previously estimated development costs incurred during the period	285,039	182,522	40,265
Accretion of discount	60,162	32,793	17,166
Other	(11,281)	(38,107)	4,171
Net change in income taxes	(71,959)	(143,166)	(72,632)
Standardized measure of discounted future net cash flows, Ending balance	$ 494,220	$ 414,381	$ 204,232

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The increase in the net discounted value of the proved oil and gas reserves due only to the passage of time.

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This is the amount that proved oil and gas reserves increased due to extensions (increased estimates in the size of proven reserves), discoveries, additions, and improved recovery (increase in ability to extract oil and gas from existing reserves).

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The increase (decrease) in the evaluation or calculation of the costs that will be required in future periods to make it possible to extract or process proved oil and gas reserves.

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The increase (decrease) in the estimate of future income taxes that will be due on future pretax net cash flows relating to the enterprise's proved oil and gas reserves.

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The change in the discounted value of the proved oil and gas reserves due to financial transactions where new reserves were purchased.

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The increase (decrease) in sales and transfer prices and in production (lifting) costs related to future production. The effects of changes in prices and costs are computed before the effects of changes in quantities.

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The actual costs incurred during the reporting period to make it possible to extract or process proved oil and gas reserves. These costs were previously estimated and reduced the estimate of discounted future cash flows from oil and gas reserves.

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Any changes made during the period to the evaluation or calculation of the estimated quantity of oil or gas in proved reserves.

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The amount of sales or transfers of oil gas that was previously included in proved oil and gas reserves. This amount is net of production costs.

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Changes in the standardized measure of discounted future net cash flows relating to proved oil and gas reserves that are not otherwise specified in the taxonomy.

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The balance as of the balance sheet date of the standardized measure of discounted future net cash flow relating to proved oil and gas reserves.

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