Summarized Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Selected Quarterly Financial Information [Abstract]
Revenues from continuing operations	$ 270,798	$ 293,141	$ 280,567	$ 257,290	$ 237,500	$ 240,374	$ 228,388	$ 273,124	$ 1,101,796	$ 979,386	$ 666,762
Gain (loss) on commodity derivatives	136,325	114,166	47,606	15,635	(20,651)	150,186	14,566	181,351	313,732	325,452	146,517
Operating Costs	184,458	160,543	151,365	145,666	144,342	128,489	125,999	124,260	642,032	523,090
Write-down of proved oil and gas properties	0	0	0	0	0	0	0	0	0	0	1,037,000
Interest expense	17,074	15,902	15,590	14,590	14,495	11,382	11,437	11,718	63,156	49,032	37,167
Litigation expense	0	0	0	0	0	0	9,902	0	0	9,902	0
Other (expense) income, net	519	(3)	(4)	20	75	12	22	151	532	260
(Loss) income from continuing operations	206,110	230,859	161,214	112,689	58,087	250,701	95,638	318,648	710,872	723,074	(696,189)
Income tax (benefit) provision	74,279	81,713	57,709	43,969	20,139	88,059	34,145	116,272	257,670	258,615	(245,136)
Net (loss) income	$ 131,831	$ 149,146	$ 103,505	$ 68,720	$ 37,948	$ 162,642	$ 61,493	$ 202,376	$ 453,202	$ 464,459	$ (451,053)
Net (loss) income per common share - basic	$ 0.86	$ 0.98	$ 0.68	$ 0.45	$ 0.25	$ 1.07	$ 0.40	$ 1.33	$ 2.97	$ 3.05	$ (2.98)
Net (loss) income per common share - fully diluted	$ 0.86	$ 0.97	$ 0.67	$ 0.44	$ 0.25	$ 1.05	$ 0.40	$ 1.31	$ 2.94	$ 3.01	$ (2.98)