Sale of Piceance and Arkoma Properties - Discontinued Operations (Detail) (USD $)	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
Results of operations and the loss on the sale of the assets
Commodity derivative fair value gains									$ 868,201,000	$ 491,689,000	$ 179,546,000
Gain (Loss) on Disposition of Other Assets									40,000,000		291,190,000
Total revenue	1,481,823,000	762,490,000	311,338,000	164,981,000	480,014,000	384,522,000	387,144,000	61,454,000	2,720,632,000	1,313,134,000	735,718,000
Total operating expenses	454,106,000	381,091,000	316,641,000	269,060,000	561,332,000	161,914,000	138,758,000	109,923,000	1,420,898,000	971,927,000	291,703,000
Lease operating									29,341,000	9,439,000	6,243,000
Gathering, compression, and transportation									461,413,000	218,428,000	91,094,000
Exploration									27,893,000	22,272,000	14,675,000
Impairment of unproved properties									15,198,000	10,928,000	12,070,000
Depletion, depreciation, and amortization									477,896,000	233,876,000	102,026,000
Accretion of asset retirement obligations									1,271,000	1,065,000	101,000
Operating income	1,027,717,000	381,399,000	(5,303,000)	(104,079,000)	(81,318,000)	222,608,000	248,386,000	(48,469,000)	1,299,734,000	341,207,000	444,015,000
Income tax (expense) benefit									(1,354,000)	(3,249,000)	272,553,000
Net income (loss) from discontinued operations			2,210,000		2,157,000	3,100,000			2,210,000	5,257,000	(510,345,000)
Piceance Basin and Arkoma Basin
Results of operations and the loss on the sale of the assets
Sales of oil, natural gas, and natural gas liquids											125,396,000
Commodity derivative fair value gains											46,358,000
Total revenue											171,754,000
Total operating expenses											158,707,000
Loss (gain) on sale of assets									(3,564,000)	(8,506,000)	795,945,000
Total expenses									(3,564,000)	(8,506,000)	954,652,000
Income (loss) from discontinued operations before income taxes									3,564,000	8,506,000	(782,898,000)
Income tax (expense) benefit									(1,354,000)	(3,249,000)	272,553,000
Net income (loss) from discontinued operations									2,210,000	5,257,000	(510,345,000)
Arkoma properties
Sale of Assets
Purchase Price											843,000,000
Results of operations and the loss on the sale of the assets
Pre-tax income received from sales tax refund and reductions in estimated expenses											(796,000,000)
Obligation assumed by purchaser	$ 3,600,000				$ 8,500,000				$ 3,600,000	$ 8,500,000