Acquisitions (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended	9 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended	1 Months Ended		9 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Apr. 30, 2012 Prize [Member]	Sep. 30, 2013 Prize [Member]	Dec. 31, 2012 2012 East Texas Acquisition [Member]	Mar. 31, 2013 2012 East Texas Acquisition [Member]	Sep. 30, 2013 2012 East Texas Acquisition [Member]	Aug. 31, 2013 2013 East Texas Acquisition [Member]	Jun. 30, 2013 2013 East Texas Acquisition [Member]	Sep. 30, 2013 2013 East Texas Acquisition [Member]	Aug. 06, 2013 2013 East Texas Acquisition [Member] MMBoe	Sep. 30, 2013 East Texas Saltwater Disposal Company [Member]
Business Acquisition [Line Items]
Date of acquisition						Apr. 20, 2012			Dec. 04, 2012			Aug. 06, 2013
Cash paid for acquisition					$ 225,000,000		$ 214,000,000			$ 107,300,000
Effective date of acquisition						Jan. 01, 2012			Nov. 01, 2012			Jun. 01, 2013
Receivable received for purchase price adjustment								2,300,000
Assumed environmental liability													500,000
Estimated proved reserves (MMBoe)													5.9
Cost associated with acquisition recorded in acquisition and transaction costs											400,000
Total revenue	126,007,000	91,128,000	336,323,000	274,588,000								4,800,000
Operating expenses	88,834,000	73,906,000	255,853,000	216,301,000								1,800,000
Percentage of voting interest acquired												56.00%		32.00%
Gain from remeasurement of previous ownership														$ 1,300,000