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, 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
Revenues and other income:
Natural gas, NGLs and oil sales	$ 448,545	$ 431,214	$ 437,678	$ 398,239	$ 398,688	$ 337,040	$ 298,349	$ 317,617	$ 1,715,676	$ 1,351,694	$ 1,173,266
Derivative fair value (loss) income	(59,355)	(40,355)	137,760	(99,875)	(5,571)	(40,728)	148,569	(60,833)	(61,825)	41,437	40,087
Gain on the sale of assets	3,162	6,008	83,287	(166)	61,836	949	(3,227)	(10,426)	92,291	49,132	2,259
Brokered natural gas, marketing and other	35,734	45,171	14,631	21,041	3,085	2,519	5,240	4,597	116,577	15,441	15,030
Total revenues and other income	428,086	442,038	673,356	319,239	458,038	299,780	448,931	250,955	1,862,719	1,457,704	1,230,642
Costs and expenses:
Direct operating	34,360	30,907	32,636	30,188	30,214	29,628	27,041	29,022	128,091	115,905	112,972
Transportation, gathering and compression	66,820	60,958	66,048	62,416	55,281	51,600	44,744	40,820	256,242	192,445	120,755
Production and ad valorem taxes	11,290	11,454	11,113	11,383	9,881	8,819	11,786	36,634	45,240	67,120	27,666
Brokered natural gas and marketing	41,692	51,117	16,662	22,315	4,994	4,887	6,491	4,062	131,786	20,434	11,986
Exploration	14,065	20,496	13,068	16,780	18,022	14,752	15,517	21,516	64,409	69,807	81,367
Abandonment and impairment of unproved properties	5,852	11,692	19,156	15,218	21,230	40,118	43,641	20,289	51,918	125,278	79,703
General and administrative	60,207	44,919	101,987	84,058	46,582	44,497	44,005	38,729	291,171	173,813	151,191
Deferred compensation plan	22,039	(2,225)	(6,878)	42,360	(14,352)	20,052	9,333	(7,830)	55,296	7,203	43,209
Interest expense	44,955	44,321	45,071	42,210	44,708	43,997	42,888	37,205	176,557	168,798	125,052
Loss on early extinguishment of debt			12,280		11,063				12,280	11,063	18,576
Depletion, depreciation and amortization	126,958	130,343	119,995	115,101	113,216	123,059	108,802	100,151	492,397	445,228	341,221
Impairment		7,012	741		34,273	1,281			7,753	35,554	38,681
Total costs and expenses	428,238	410,994	431,879	442,029	375,112	382,690	354,248	320,598	1,713,140	1,432,648	1,152,379
(Loss) income from continuing operations before income taxes	(152)	31,044	241,477	(122,790)	82,926	(82,910)	94,683	(69,643)	149,579	25,056	78,263
Income tax (benefit) expense	(143)		(25)	25	(1,778)				(143)	(1,778)	637
Deferred	(28,180)	11,866	97,519	(47,205)	31,742	(29,074)	39,007	(27,843)	34,000	13,832	34,920
Total income tax expense	(28,323)	11,866	97,494	(47,180)	29,964	(29,074)	39,007	(27,843)	33,857	12,054	35,557
Net income	$ 28,171	$ 19,178	$ 143,983	$ (75,610)	$ 52,962	$ (53,836)	$ 55,676	$ (41,800)	$ 115,722	$ 13,002	$ 58,026
Net (loss) income per common share:
Basic	$ 0.17	$ 0.12	$ 0.88	$ (0.47)	$ 0.33	$ (0.34)	$ 0.34	$ (0.26)	$ 0.71	$ 0.08	$ 0.36
Diluted	$ 0.17	$ 0.12	$ 0.88	$ (0.47)	$ 0.32	$ (0.34)	$ 0.34	$ (0.26)	$ 0.70	$ 0.08	$ 0.36