Acquisition and Development Agreement and Carry and Earning Agreement (Details) - Mitsui E&P Texas LP $ in Millions	Jun. 30, 2011 USD ($)
Acquisition and Development Agreement [Abstract]
Percentage of Working Interests Transferred to Acquirer Entity	12.50%
Acreage of Working Interests Transferred to Acquirer Entity	39,000
Percentage of the entity's costs and expenses during the first three years following the closing of the transaction, borne by the acquirer entity (as a percent)	90.00%
Costs and expenses incurred by acquiree entity on behalf of entity that the acquirer entity has agreed to pay	$ 680.0
Portion of Drilling and Completion Costs Not Carried by Acquirer	10.00%