Dispositions and Acquisitions - Additional Information (Detail) (USD $)	12 Months Ended				3 Months Ended	9 Months Ended		12 Months Ended		6 Months Ended	12 Months Ended		3 Months Ended		12 Months Ended	3 Months Ended	1 Months Ended	9 Months Ended	1 Months Ended	12 Months Ended	6 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Permian Basin Assets [Member]	Sep. 30, 2011 Pennsylvania [Member]	Sep. 30, 2012 Pennsylvania [Member]	Sep. 30, 2011 Pennsylvania [Member]	Dec. 31, 2012 Pennsylvania [Member]	Dec. 31, 2011 Pennsylvania [Member]	Jun. 30, 2011 Barnett Shale properties [Member]	Dec. 31, 2012 Barnett Shale properties [Member]	Dec. 31, 2011 Barnett Shale properties [Member]	Mar. 31, 2012 East Texas [Member]	Sep. 30, 2011 East Texas [Member]	Dec. 31, 2011 East Texas [Member]	Mar. 31, 2011 Low pressure pipeline [Member]	Mar. 31, 2010 Ohio properties [Member]	Sep. 30, 2010 Ohio properties [Member]	Jun. 30, 2010 Virginia properties [Member]	Dec. 31, 2012 Oklahoma [Member]	Jun. 30, 2012 Marcellus Shale properties [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gross cash proceeds from sale of oil and gas properties	$ 168,219,000	$ 53,926,000	$ 327,765,000		$ 5,400,000	$ 13,900,000							$ 8,600,000	$ 10,500,000			$ 323,000,000			$ 135,000,000	$ 2,500,000
Gain (Loss) on sale of oil and gas properties	49,132,000	2,259,000	76,642,000		0	746,000	0						(10,900,000)							55,200,000	(2,500,000)
Gain on exchange of acreage								6,800,000	4,500,000
Overriding royalty in the prospect minimum						1.00%							2.50%
Overriding royalty in the prospect maximum						5.00%							5.00%
Sale of oil and gas properties													75.00%
Estimated sale of Permian Basin properties				275,000,000
Gross cash proceeds											889,300,000
Gain or loss on sale of natural gas properties											4,800,000
Proceeds from derivative contract										40,000,000
Loss on derivative contracts										(1,700,000)
Pretax gain												25,100,000
Proceeds from exchange of unproved property									11,500,000
Impairment of proved properties	34,273,000	38,681,000	6,505,000											31,200,000	31,200,000
Proceeds from sale of pipeline																14,700,000
Gain (Loss) on sale of pipeline																0
Percentage of Ohio sale closed																	90.00%
Gain from sale of oil and gas properties																	77,600,000
Proceeds used to repay outstanding borrowings under credit facility	1,221,000,000	974,826,000	1,105,000,000															135,000,000
Acquisitions of producing and unproved properties																			134,500,000
Purchase price allocated to proved properties																			131,300,000
Acquisitions of unproved property			$ 3,697,000																$ 3,700,000