Summarized Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
Revenues from continuing operations	$ 319,050	$ 288,608	$ 296,063	$ 326,299	$ 225,197	$ 221,205	$ 261,376	$ 225,626	$ 1,230,020	$ 933,404	$ 809,974
Gain (loss) on commodity derivatives	110,725	32,052	(15,102)	(45,273)	(26,204)	2,074	22,091	(44,715)	82,402	(46,754)	73,581
Expenses from continuing operations	195,083	169,669	150,850	154,829	141,753	136,389	143,002	139,994	670,431	561,138
Ceiling test and other impairments					0	0	0	0	0	0	2,972,464
Interest expense	42,196	29,599	27,294	27,068	25,310	25,174	25,238	25,764	126,157	101,486	88,180
Gain on sale of property	8,022	0	0	0					8,022	0	0
Contract Cancellation Fees					0	0	0	0	0	0	15,469
Other (expense) income, net	5,311	2,582	2,688	2,590	2,332	2,575	2,641	2,648	13,171	10,196
Income (loss) from continuing operations	205,829	123,974	105,505	101,719	34,262	64,291	117,868	17,801	537,027	234,222	(2,877,111)
Income tax (benefit)	(3,901)	(1,383)	(544)	4	(6,856)	381	1,491	1,368	(5,824)	(3,616)	(700,213)
Net income (loss)	$ 209,730	$ 125,357	$ 106,049	$ 101,715	$ 41,118	$ 63,910	$ 116,377	$ 16,433	$ 542,851	$ 237,838	$ (2,176,898)
Net (income (loss) per common share - basic	$ 1.37	$ 0.82	$ 0.69	$ 0.66	$ 0.27	$ 0.42	$ 0.76	$ 0.11	$ 3.54	$ 1.55	$ (14.24)
Net income (loss) per common share - fully diluted	$ 1.36	$ 0.81	$ 0.68	$ 0.66	$ 0.27	$ 0.41	$ 0.75	$ 0.11	$ 3.51	$ 1.54	$ (14.24)