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Acquisition and Development Agreement (Details) (USD $)
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Jun. 30, 2012
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Dec. 31, 2011
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Jun. 30, 2012
Mitsui E&P Texas LP
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Jun. 30, 2011
Mitsui E&P Texas LP
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| Acquisition and development agreement | ||||
| Percentage of working interests transferred to acquiree entity (as a percent) | 12.50% | |||
| Acreage of working interests transferred to acquiree entity (in acres) | 39,000 | |||
| Percentage of the entity's costs and expenses during the first three years following the closing of the transaction, borne by the acquiree entity (as a percent) | 90.00% | |||
| Costs and expenses incurred by acquiree entity on behalf of entity that the acquiree entity has agreed to pay | $ 680,000,000 | |||
| RetainedShareOfDrillingAndCompletionCosts | 10.00% | |||
| Restricted Cash and Cash Equivalents, Noncurrent | 109,486,000 | 124,703,000 | ||
| Utilization Amount of Carry and Earning Agreement | $ 128,300,000 |