Selected Quarterly Financial Data (Unaudited) - Selected Quarterly Financial Data (Detail) (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
Revenues and other income:
Natural gas, NGLs and oil sales	$ 416,388	$ 446,067	$ 477,517	$ 572,017	$ 448,545	$ 431,214	$ 437,678	$ 398,239	$ 1,911,989	$ 1,715,676	$ 1,351,694
Derivative fair value (loss) income	412,422	142,057	(24,109)	(146,850)	(59,355)	(40,355)	137,760	(99,875)	383,520	(61,825)	41,437
(Loss) gain on the sale of assets	3,760	167	282,064	(353)	3,162	6,008	83,287	(166)	285,638	92,291	49,132
Brokered natural gas, marketing and other	39,644	28,324	30,052	32,528	35,734	45,171	14,631	21,041	130,548	116,577	15,441
Total revenues and other income	872,214	616,615	765,524	457,342	428,086	442,038	673,356	319,239	2,711,695	1,862,719	1,457,704
Costs and expenses:
Direct operating	37,961	37,792	34,935	39,795	34,360	30,907	32,636	30,188	150,483	128,091	115,905
Transportation, gathering and compression	89,542	84,777	76,809	74,161	66,820	60,958	66,048	62,416	325,289	256,242	192,445
Production and ad valorem taxes	11,923	10,110	10,844	11,678	11,290	11,454	11,113	11,383	44,555	45,240	67,120
Brokered natural gas and marketing	32,370	28,706	34,775	34,129	41,692	51,117	16,662	22,315	129,980	131,786	20,434
Exploration	23,638	11,443	13,621	14,846	14,065	20,496	13,068	16,780	63,548	64,409	69,807
Abandonment and impairment of unproved properties	14,308	13,444	9,332	9,995	5,852	11,692	19,156	15,218	47,079	51,918	125,278
General and administrative	52,363	54,963	56,888	49,212	60,207	44,919	101,987	84,058	213,426	291,171	173,813
Termination costs	8,371								8,371
Deferred compensation plan	(36,836)	(46,198)	10,519	(2,035)	22,039	(2,225)	(6,878)	42,360	(74,550)	55,296	7,203
Interest expense	38,900	39,188	45,488	45,401	44,955	44,321	45,071	42,210	168,977	176,557	168,798
Loss on early extinguishment of debt			24,596				12,280		24,596	12,280	11,063
Depletion, depreciation and amortization	146,539	142,450	133,361	128,682	126,958	130,343	119,995	115,101	551,032	492,397	445,228
Impairment of proved properties and other assets	3,033		24,991			7,012	741		28,024	7,753	35,554
Total costs and expenses	422,112	376,675	476,159	405,864	428,238	410,994	431,879	442,029	1,680,810	1,713,140	1,432,648
Income before income taxes	450,102	239,940	289,365	51,478	(152)	31,044	241,477	(122,790)	1,030,885	149,579	25,056
Income tax expense (benefit):
Current	(4)		(1)	6	(143)		(25)	25	1	(143)	(1,778)
Deferred	166,052	93,522	117,977	18,951	(28,180)	11,866	97,519	(47,205)	396,502	34,000	13,832
Total income tax expense	166,048	93,522	117,976	18,957	(28,323)	11,866	97,494	(47,180)	396,503	33,857	12,054
Net income	$ 284,054	$ 146,418	$ 171,389	$ 32,521	$ 28,171	$ 19,178	$ 143,983	$ (75,610)	$ 634,382	$ 115,722	$ 13,002
Net (loss) income per common share:
Basic	$ 1.68	$ 0.87	$ 1.04	$ 0.20	$ 0.17	$ 0.12	$ 0.88	$ (0.47)	$ 3.81	$ 0.71	$ 0.08
Diluted	$ 1.68	$ 0.86	$ 1.04	$ 0.20	$ 0.17	$ 0.12	$ 0.88	$ (0.47)	$ 3.79	$ 0.70	$ 0.08