Summarized Quarterly Financial Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Financial Information [Abstract]
Revenues from continuing operations	$ 225,197,000	$ 221,205,000	$ 261,376,000	$ 225,626,000	$ 217,186,000	$ 196,375,000	$ 170,270,000	$ 226,143,000	$ 933,404,000	$ 809,974,000	$ 1,101,796,000
Gain (loss) on commodity derivatives	(26,204,000)	2,074,000	22,091,000	(44,715,000)	(3,519,000)	(9,896,000)	(33,287,000)	120,283,000	(46,754,000)	73,581,000	313,732,000
Expenses from continuing operations	141,753,000	136,389,000	143,002,000	139,994,000	146,682,000	156,503,000	186,064,000	193,539,000	561,138,000	682,788,000
Ceiling test and other impairments					496,501,000	606,827,000	1,869,136,000	0	0	2,972,464,000	0
Interest expense	25,310,000	25,174,000	25,238,000	25,764,000	25,765,000	25,369,000	18,748,000	18,298,000	101,486,000	88,180,000	63,156,000
Contract Cancellation Fees					6,248,000	(291,000)	4,666,000	4,846,000	0	15,469,000	0
Other (expense) income, net	2,332,000	2,575,000	2,641,000	2,648,000	(1,738,000)	(42,000)	7,000	8,000	10,196,000	(1,765,000)
(Loss) income from continuing operations	34,262,000	64,291,000	117,868,000	17,801,000	(463,267,000)	(601,971,000)	(1,941,624,000)	129,751,000	234,222,000	(2,877,111,000)	710,872,000
Income tax (benefit) provision	(6,856,000)	381,000	1,491,000	1,368,000	8,765,000	175,000	(754,642,000)	45,489,000	(3,616,000)	(700,213,000)	257,670,000
Net (loss) income	$ 41,118,000	$ 63,910,000	$ 116,377,000	$ 16,433,000	$ (472,032,000)	$ (602,146,000)	$ (1,186,982,000)	$ 84,262,000	$ 237,838,000	$ (2,176,898,000)	$ 453,202,000
Net (loss) income per common share - basic	$ 0.27	$ 0.42	$ 0.76	$ 0.11	$ (3.09)	$ (3.94)	$ (7.76)	$ 0.55	$ 1.55	$ (14.24)	$ 2.97
Net (loss) income per common share - fully diluted	$ 0.27	$ 0.41	$ 0.75	$ 0.11	$ (3.09)	$ (3.94)	$ (7.76)	$ 0.55	$ 1.54	$ (14.24)	$ 2.94