Discontinued Operations - Components of Discontinued Operations (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Natural gas, NGLs and oil sales	$ 337,040,000	$ 304,230,000	$ 953,006,000	$ 841,546,000
Other	(2,368,000)	442,000	(3,084,000)	357,000
Gain on the sale of assets	949,000	203,000	(12,704,000)	(1,280,000)
Total revenues and other income	294,893,000	370,636,000	984,226,000	918,589,000
Direct operating	29,628,000	29,828,000	85,691,000	87,054,000
Transportation, gathering and compression	51,600,000	32,431,000	137,164,000	86,179,000
Production and ad valorem taxes	8,819,000	7,317,000	57,239,000	21,746,000
Exploration expense	14,752,000	17,606,000	51,785,000	56,385,000
Interest expense	43,997,000	34,181,000	124,090,000	90,343,000
Depletion, depreciation and amortization	123,059,000	93,619,000	332,012,000	244,129,000
Total costs and expenses	377,803,000	314,910,000	1,042,096,000	837,712,000
Income tax expense
Current		(7,000)		1,000
Deferred	(29,074,000)	22,547,000	(17,910,000)	35,345,000
Income Tax Expense Benefit	(29,074,000)	22,540,000	(17,910,000)	35,346,000
Net income from discontinued operations		1,569,000		15,484,000
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Natural gas, NGLs and oil sales		1,673,000		58,997,000
Other				10,000
Gain on the sale of assets		1,032,000		4,852,000
Total revenues and other income		2,705,000		63,859,000
Direct operating		(611,000)		9,835,000
Transportation, gathering and compression		950,000		5,240,000
Production and ad valorem taxes		(44,000)		1,206,000
Exploration expense				37,000
Interest expense				14,791,000	[1]
Depletion, depreciation and amortization				8,894,000
Total costs and expenses		295,000		40,003,000
Income before income taxes		2,410,000		23,856,000
Income tax expense
Current
Deferred		841,000		8,372,000
Income Tax Expense Benefit		841,000		8,372,000
Net income from discontinued operations		$ 1,569,000		$ 15,484,000

[1] Interest expense is allocated to discontinued operations based on the ratio of net assets of discontinued operations to our consolidated net assets plus long-term debt.