Selected Quarterly Financial Data (Unaudited) - Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues and other income:
Natural gas, NGLs and oil sales	$ 254,043	$ 252,065	$ 258,053	$ 325,483	$ 416,388	$ 446,067	$ 477,517	$ 572,017	$ 1,089,644	$ 1,911,989	$ 1,715,676
Derivative fair value income (loss)	126,312	202,004	(34,791)	122,839	412,422	142,057	(24,109)	(146,850)	416,364	383,520	(61,825)
Brokered natural gas, marketing and other	30,372	25,864	21,339	14,485	39,644	28,324	30,052	32,528	92,060	130,548	116,577
Total revenues and other income	410,727	479,933	244,601	462,807	868,454	616,448	483,460	457,695	1,598,068	2,426,057	1,770,428
Costs and expenses:
Direct operating	29,388	35,058	34,780	37,137	37,961	37,792	34,935	39,795	136,363	150,483	128,091
Transportation, gathering and compression	112,481	99,634	95,198	89,426	89,542	84,777	76,809	74,161	396,739	325,289	256,242
Production and ad valorem taxes	7,354	7,336	9,242	9,928	11,923	10,110	10,844	11,678	33,860	44,555	45,240
Brokered natural gas and marketing	34,942	32,331	27,031	21,562	32,370	28,706	34,775	34,129	115,866	129,980	131,786
Exploration	4,260	4,235	5,025	7,886	23,638	11,443	13,621	14,846	21,406	63,548	64,409
Abandonment and impairment of unproved properties	11,432	12,366	12,330	11,491	14,308	13,444	9,332	9,995	47,619	47,079	51,918
General and administrative	43,544	46,178	55,964	48,329	52,363	54,963	56,888	49,212	194,015	213,426	291,171
Termination costs	8,780	(77)	417	5,950	8,371				15,070	8,371	0
Deferred compensation plan	(21,016)	(43,705)	(7,282)	(5,624)	(36,836)	(46,198)	10,519	(2,035)	(77,627)	(74,550)	55,296
Interest	40,849	42,904	43,479	39,207	38,900	39,188	45,488	45,401	166,439	168,977	176,557
Loss on early extinguishment of debt		22,495					24,596		22,495	24,596	12,280
Depletion, depreciation and amortization	127,977	153,993	151,895	147,290	146,539	142,450	133,361	128,682	581,155	551,032	492,397
Impairment of proved properties and other assets	87,941	502,233			3,033		24,991		590,174	28,024	7,753
Loss (gain) on the sale of assets	408,909	681	(2,909)	175	(3,760)	(167)	(282,064)	353	406,856	(285,638)	(92,291)
Total costs and expenses	896,841	915,662	425,170	412,757	418,352	376,508	194,095	406,217	2,650,430	1,395,172	1,620,849
(Loss) income before income taxes	(486,114)	(435,729)	(180,569)	50,050	450,102	239,940	289,365	51,478	(1,052,362)	1,030,885	149,579
Income tax (benefit) expense:
Current	29				(4)		(1)	6	29	1	(143)
Deferred	(164,316)	(134,781)	(61,975)	22,366	166,052	93,522	117,977	18,951	(338,706)	396,502	34,000
Total income tax expense	(164,287)	(134,781)	(61,975)	22,366	166,048	93,522	117,976	18,957	(338,677)	396,503	33,857
Net (loss) income	$ (321,827)	$ (300,948)	$ (118,594)	$ 27,684	$ 284,054	$ 146,418	$ 171,389	$ 32,521	$ (713,685)	$ 634,382	$ 115,722
Net income (loss) per common share:
Basic	$ (1.93)	$ (1.81)	$ (0.71)	$ 0.16	$ 1.68	$ 0.87	$ 1.04	$ 0.20	$ (4.29)	$ 3.81	$ 0.71
Diluted	$ (1.93)	$ (1.81)	$ (0.71)	$ 0.16	$ 1.68	$ 0.86	$ 1.04	$ 0.20	$ (4.29)	$ 3.79	$ 0.70