Acquisitions (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Apr. 17, 2012	Jun. 30, 2012 Prize [Member]	Jun. 30, 2012 Prize [Member]	Apr. 20, 2012 Prize [Member]	Dec. 31, 2011 Prize [Member]
Date of acquisition							Apr. 20, 2012
Cash paid for acquisition								$ 225,000,000
Effective date of acquisition							Jan. 01, 2012
Estimated proved reserves (BOE)									13,300,000
Cost associated with acquisition recorded in general and administrative expenses						600,000	600,000
Costs associated with the Offering					500,000
Estimated environmental liability						1,900,000	1,900,000	1,891,000
Revenue	62,209,000	67,880,000	127,996,000	130,711,000		6,500,000	6,500,000
Operating expenses	$ 56,865,000	$ 49,015,000	$ 108,748,000	$ 94,919,000		$ 2,700,000	$ 2,700,000