Property and Equipment (Narr) (Acquisitions) (Details) (USD $)	3 Months Ended								12 Months Ended			0 Months Ended	12 Months Ended					12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	May 31, 2012 [AlbaFieldMember]	Dec. 31, 2012 [AlbaFieldMember]	Dec. 31, 2012 [HaynesvilleAcreageMember]	Dec. 31, 2012 [MarcellusAcreageMember]	Dec. 31, 2010 [MarcellusAcreageMember]	Jun. 30, 2010 [MarcellusAcreageMember]	Dec. 31, 2012 [CygnusMember]	Dec. 31, 2012 [BacchusMember]	Feb. 23, 2011 [BacchusMember]	Dec. 31, 2012 [BacchusMember] [CapitalCostIncurredBySellerMember]	Dec. 31, 2012 [AlabamaMember]
Property, Plant and Equipment [Line Items]
Impairment of oil and gas properties	$ 6,000,000	$ 11,400,000	$ 20,000,000	$ 15,700,000	$ 36,900,000	$ 28,800,000	$ 0	$ 0	$ 53,072,000	$ 65,706,000	$ 7,692,000
Proceeds from sale of oil and gas property and equipment									1,407,000	0	108,316,000							110,000,000
Gain (loss) on sale of oil and gas property																		87,000,000
Business Acquisition Date Of Acquisition Agreement 1													May 31, 2012		Oct. 26, 2012			Oct. 19, 2010	Feb. 23, 2011
Additional working interest												23.43%							20.00%
Ownership percentage of producing acreage												25.68%		50.00%					30.00%
Business Acquisition, Cost of Acquired Entity, Cash Paid												$ 229,577,000				$ 15,000,000	$ 7,500,000		$ 6,200,000	$ 9,200,000	$ 9,400,000	$ 8,000,000