Dispositions - Additional Information (Detail) (USD $)	9 Months Ended					6 Months Ended	12 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Southern Oklahoma [Member] Well	Sep. 30, 2012 Pennsylvania [Member]	Sep. 30, 2011 Pennsylvania [Member]	Jun. 30, 2011 Barnett Shale Properties [Member]	Dec. 31, 2011 Barnett Shale Properties [Member]	Mar. 31, 2011 Low pressure pipeline [Member]	Jun. 30, 2012 Marcellus Shale Properties [Member]	Mar. 31, 2012 East Texas [Member]	Sep. 30, 2011 East Texas [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of oil and gas properties	$ 32,082,000	$ 40,264,000		$ 13,900,000	$ 5,400,000				$ 2,500,000	$ 8,600,000	$ 10,500,000
Gain (Loss) on sale of oil and gas properties	(12,704,000)	(1,280,000)		746,000	0				(2,500,000)	(10,900,000)
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%
Number of productive wells			45
Purchase price			135,000,000
Impairment on oil and gas properties											31,200,000
Proceeds from the sale of natural gas properties							889,300,000
Proceeds from sale of pipeline								14,700,000
Gain from sale of pipeline								0
Loss on derivative contracts						$ 1,700,000