Acquisition and Dispositions of Oil and Gas Properties (Details) (USD $)	3 Months Ended									12 Months Ended				12 Months Ended			3 Months Ended														0 Months Ended										0 Months Ended					0 Months Ended	12 Months Ended	0 Months Ended			1 Months Ended	0 Months Ended	12 Months Ended	0 Months Ended
	Dec. 31, 2011 mmboe		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010 mmboe	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011 mmboe	Dec. 31, 2010 mmboe	Dec. 31, 2009 mmboe	Dec. 31, 2008 mmboe	Dec. 31, 2010 Alabama Shale	Dec. 31, 2009 Alabama Shale	Dec. 31, 2010 Alabama Shale Well	Dec. 31, 2009 Chattanooga Shale Well	Dec. 31, 2009 Chattanooga Shale, Chesapeake Well	Dec. 31, 2011 Oil Reserves MBbls	Dec. 31, 2010 Oil Reserves MBbls	Dec. 31, 2009 Oil Reserves MBbls	Dec. 31, 2008 Oil Reserves MBbls	Dec. 31, 2011 Natural Gas Liquids Reserves MBbls	Dec. 31, 2010 Natural Gas Liquids Reserves MBbls	Dec. 31, 2009 Natural Gas Liquids Reserves MBbls	Dec. 31, 2008 Natural Gas Liquids Reserves MBbls	Dec. 31, 2011 Natural Gas Reserves MMcf	Dec. 31, 2010 Natural Gas Reserves MMcf	Dec. 31, 2009 Natural Gas Reserves MMcf	Dec. 31, 2008 Natural Gas Reserves MMcf	Dec. 27, 2011 Dec 27, 2011, Permian Basin mmboe	Dec. 27, 2011 Dec 27, 2011, Permian Basin Oil Reserves	Dec. 27, 2011 Dec 27, 2011, Permian Basin Natural Gas Liquids Reserves	Dec. 27, 2011 Dec 27, 2011, Permian Basin Natural Gas Reserves	Nov. 16, 2011 Nov 16, 2011, Permian Basin mmboe	Nov. 16, 2011 Nov 16, 2011, Permian Basin Oil Reserves	Nov. 16, 2011 Nov 16, 2011, Permian Basin Natural Gas Liquids Reserves	Nov. 16, 2011 Nov 16, 2011, Permian Basin Natural Gas Reserves	Dec. 31, 2011 July 2011, Permian Basin	Dec. 31, 2011 April 2011, Permian Basin A	Dec. 15, 2010 Dec 15, 2010, Permian Basin mmboe	Dec. 15, 2010 Dec 15, 2010, Permian Basin Oil Reserves	Dec. 15, 2010 Dec 15, 2010, Permian Basin Natural Gas Liquids Reserves	Dec. 15, 2010 Dec 15, 2010, Permian Basin Natural Gas Reserves	Dec. 09, 2010 Dec 9, 2010, Permian Basin Boe	Sep. 30, 2010 Sep 30, 2010, Permian Basin mmboe	Dec. 31, 2010 Sep 30, 2010, Permian Basin	Sep. 30, 2010 Sep 30, 2010, Permian Basin Oil Reserves	Sep. 30, 2010 Sep 30, 2010, Permian Basin Natural Gas Liquids Reserves	Sep. 30, 2010 Sep 30, 2010, Permian Basin Natural Gas Reserves	Sep. 30, 2009 September 2009, Permian Basin	Jun. 30, 2009 Jun 30, 2009, Permian Basin mmboe	Dec. 31, 2009 Jun 30, 2009, Permian Basin	Jun. 30, 2009 Jun 30, 2009, Permian Basin Oil Reserves	Jun. 30, 2009 Jun 30, 2009, Permian Basin Natural Gas Liquids Reserves	Jun. 30, 2009 Jun 30, 2009, Permian Basin Natural Gas Reserves
Significant Acquisitions and Disposals [Line Items]
Consideration - Purchase Price																															$ 55,994,000				$ 162,092,000				$ 20,000,000	$ 37,000,000	$ 73,630,000				$ 103,000,000	$ 188,314,000						$ 181,249,000
Net acres																																								11,000
Proved reserves acquired	343.1					302.9				343.1	302.9	257.8	264.1						129,578	103,262	77,963	62,034	53,957	40,601	30,257	28,953	957,368	954,387	897,546	1,038,453	3.4				13.6						7.6				0	18						15.2
Percentage of undeveloped portion of proved reserves										69.00%	77.00%	81.00%																			77.00%				76.00%						92.00%					89.00%						24.00%
Acquired proved reserves by type of reserve, percentage																																61.00%	24.00%	15.00%		59.00%	25.00%	16.00%				62.00%	24.00%	14.00%				65.00%	22.00%	13.00%				76.00%	16.00%	8.00%
Gain (loss) in other operating revenues from sale of oil properties, pre-tax																																																			4,900,000
Cash received from sale of oil properties																																																			6,500,000
Exploration Abandonment and Impairment Expense														39,700,000	2,100,000	15,500,000	5,600,000	1,200,000
Recognized amounts of identifiable assets acquired and liabilities assumed
Proved properties																															32,045,000				151,544,000						41,066,000					151,747,000						182,668,000
Unproved leasehold properties																															23,686,000				7,883,000						32,500,000					35,360,000						3,800,000
Accounts receivable																															680,000				3,151,000						143,000					1,461,000						4,987,000
Inventory and other																																																				455,000
Accounts payable																															(244,000)				(344,000)											(112,000)						(947,000)
Asset retirement obligation																															(173,000)				(142,000)						(79,000)					(142,000)						(6,590,000)
Environmental liabilities																																																				(3,124,000)
Total identifiable net assets																															55,994,000				162,092,000						73,630,000					188,314,000						181,249,000
Operating revenues	288,148,000	[1]	378,568,000	330,399,000	486,364,000	374,091,000	295,804,000	333,725,000	574,914,000	1,483,479,000	1,578,534,000	1,440,420,000																																			5,000,000						22,300,000
Operating income	31,641,000	[1]	150,412,000	106,335,000	159,881,000	135,566,000	67,320,000	98,293,000	192,198,000	448,269,000	493,377,000	435,393,000																																			2,100,000						8,900,000
Oil and Gas Property Acquisition, Pro Forma Information [Abstract]
Operating revenue																																																					1,458,995,000
Operating income																																																					$ 439,624,000

[1] The three months ended December 31, 2011 includes an after-tax non-cash mark-to-market loss on derivatives of $56.6 million, or $0.78 per diluted share.