Acquisition and Development Agreement and Carry and Earning Agreement (Details) (Mitsui E&P Texas LP, USD $) In Millions, unless otherwise specified	Jun. 30, 2011
Mitsui E&P Texas LP
Acquisition and Development Agreement [Abstract]
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	$ 680.0
Portion of Drilling and Completion Costs Not Carried by Acquirer	10.00%