Suspended Exploratory Well Costs (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Suspended Exploratory Well Costs
Beginning balance at January 1	$ 23,908	$ 19,052
Additions to capitalized exploratory well costs pending the determination of proved reserves	35,463	28,897
Reclassifications based on determination of proved reserves	(11,619)	(24,041)
Capitalized exploratory well costs charged to expense	0	0
Balance at end of period	47,752	23,908
Less exploratory well costs that have been capitalized for a period of one year or less	(42,519)	(13,181)
Capitalized exploratory well costs that have been capitalized for a period greater than one year	$ 5,233	$ 10,727
Number of projects that have exploratory well costs that have been capitalized for a period greater than one year	3	4

Additions to capitalized exploratory well costs that were added during the reporting period pending the determination of proved reserves.

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Capitalized exploratory well costs that were expensed during the year because proved reserves were not found at the exploratory well.

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The amount of capitalized exploratory well costs that is pending the determination of proved reserves. This does not include amounts that were capitalized and subsequently expensed in the same annual period.

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The amount of exploratory well costs that have been capitalized for a period of greater than one year after the completion of drilling.

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The amount of exploratory well costs that have been capitalized for a period of one year or less after the completion of drilling.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The number of projects for which exploratory well costs have been capitalized for a period of greater than one year after the completion of drilling.

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Previously capitalized costs of the drilling of wells that were reclassified as part of the costs of the enterprise's wells and related equipment and facilities at the time that proved reserves were found.

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