Acquisitions (Narrative) (Details) (USD $)	0 Months Ended	12 Months Ended					12 Months Ended			12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Prize [Member]	Apr. 20, 2012 Prize [Member]	Dec. 31, 2011 Prize [Member] MMBoe	Dec. 31, 2012 East Texas Oil Field Properties [Member]	Dec. 04, 2012 East Texas Oil Field Properties [Member]	Dec. 31, 2011 East Texas Oil Field Properties [Member] MMBoe	Dec. 31, 2012 December 2012 Transferred Properties [Member]	Dec. 28, 2012 December 2012 Transferred Properties [Member]	Dec. 31, 2012 October 2011 Transferred Properties [Member]	Oct. 31, 2011 October 2011 Transferred Properties [Member]	Dec. 31, 2012 Denbury Properties [Member]	Dec. 31, 2010 Denbury Properties [Member]	May 14, 2010 Denbury Properties [Member] MMBoe
Business Acquisition [Line Items]
Date of acquisition		Apr. 20, 2012		Apr. 20, 2012			Dec. 04, 2012			Dec. 28, 2012		Oct. 01, 2011		May 14, 2010
Cash paid for acquisition					$ 225,100,000			$ 214,306,000
Effective date of acquisition				Jan. 01, 2012			Nov. 01, 2012
Estimated proved reserves (MMBoe)						13.3			10.8							77
Cost associated with acquisition recorded in acquisition and transaction costs				1,100,000			300,000
Estimated environmental liability				1,900,000	1,891,000
Revenue	8,986,000	371,998,000	360,904,000	24,600,000			3,100,000								253,386,000
Operating expenses	7,141,000	301,657,000	269,128,000	8,300,000			1,100,000
Aggregate purchase price											$ 28,600,000		$ 578,800,000			$ 893,000,000