Acquisition and Development Agreement and Carry and Earning Agreement (Details) (USD $)			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Mitsui E&P Texas LP	Jun. 30, 2011 Mitsui E&P Texas LP	Apr. 29, 2010 Third Party Acquirer A
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				$ 680,000,000
Acquisition and Development Agreement, Estimated Time to Complete, Low End of Range			2
Portion of Drilling and Completion Costs Not Carried by Acquirer			10.00%
Restricted cash	86,773,000	124,703,000
Carry Amount, Portion Used to Date			277,500,000
Carry and Earning Agreement Disclosure [Abstract]
Net acreage transfered from Carry and Earning Agreement					8,400
Interest earned through Carry and Earning Agreement for 23,400 net acres					5.00%
Acreage that working interest can be earned					23,400
Third party total investment					$ 91,300,000
Interest earned through Carry and Earning Agreement for 8,400 net acres					95.00%