Capitalized Costs Relating to Oil and Gas Producing Activities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]
Proved properties	$ 277,987	$ 293,486	$ 353,386
Unproved properties	120,288	171,303	73,067
Wells, equipment and facilities	2,081,103	1,840,154	1,527,749
Support equipment	6,645	6,254	5,938
Capitalized Costs, Oil and Gas Producing Activities, Gross, Total	2,486,023	2,311,197	1,960,140
Accumulated depreciation and depletion	(710,948)	(609,380)	(487,106)
Net capitalized costs	$ 1,775,075	$ 1,701,817	$ 1,473,034

Capitalized costs of proved properties incurred for any combination mineral interests acquisitions; wells and related equipment; support equipment and facilities; and uncompleted wells and equipment and other costs not previously disclosed within this table.

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Capitalized costs incurred for support equipment and facilities used in oil and gas producing activities (such as seismic equipment, drilling equipment, construction and grading equipment, vehicles, repair shops, warehouses, supply points, camps, and division, district, or field offices).

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- Definition

Capitalized costs of unproved properties incurred for any combination mineral interests acquisitions and other costs not previously disclosed within this table.

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