Summarized Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 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
Selected Quarterly Financial Information [Abstract]
Revenues from continuing operations	$ 217,186	$ 196,375	$ 170,270	$ 226,143	$ 270,798	$ 293,141	$ 280,567	$ 257,290	$ 809,974	$ 1,101,796	$ 979,386
Gain (loss) on commodity derivatives	(3,519)	(9,896)	(33,287)	120,283	136,325	114,166	47,606	15,635	73,581	313,732	325,452
Expenses from continuing operations	146,682	156,503	186,064	193,539	184,458	160,543	151,365	145,666	682,788	642,032
Ceiling test and other impairments	496,501	606,827	1,869,136	0	0	0	0	0	2,972,464	0	0
Interest expense	25,765	25,369	18,748	18,298	17,074	15,902	15,590	14,590	88,180	63,156	49,032
Contract Cancellation Fees	6,248	(291)	4,666	4,846					15,469	0	0
Litigation expense					0	0	0	0	0	0	9,902
Other (expense) income, net	(1,738)	(42)	7	8	519	(3)	(4)	20	(1,765)	532
(Loss) income from continuing operations	(463,267)	(601,971)	(1,941,624)	129,751	206,110	230,859	161,214	112,689	(2,877,111)	710,872	723,074
Income tax (benefit) provision	8,765	175	(754,642)	45,489	74,279	81,713	57,709	43,969	(700,213)	257,670	258,615
Net (loss) income	$ (472,032)	$ (602,146)	$ (1,186,982)	$ 84,262	$ 131,831	$ 149,146	$ 103,505	$ 68,720	$ (2,176,898)	$ 453,202	$ 464,459
Net (loss) income per common share - basic	$ (3.09)	$ (3.94)	$ (7.76)	$ 0.55	$ 0.86	$ 0.98	$ 0.68	$ 0.45	$ (14.24)	$ 2.97	$ 3.05
Net (loss) income per common share - fully diluted	$ (3.09)	$ (3.94)	$ (7.76)	$ 0.55	$ 0.86	$ 0.97	$ 0.67	$ 0.44	$ (14.24)	$ 2.94	$ 3.01