Dispositions - Additional Information (Detail) (USD $)	3 Months Ended				3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Permian Basin Assets	Feb. 26, 2013 Permian Basin Assets	Mar. 31, 2012 East Texas	Mar. 31, 2013 Pennsylvania
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Agreed upon sales price of Permian Basin properties				$ 275,000,000
Gain (Loss) on sale of oil and gas properties	(166,000)	(10,426,000)			(10,900,000)
Expected gain on disposition			84,000,000
Sale of oil and gas properties					75.00%
Proceeds from disposal of assets	$ 38,196,000	$ 9,852,000	$ 27,500,000		$ 8,600,000	$ 10,000,000
Overriding royalty in the prospect minimum					2.50%
Overriding royalty in the prospect maximum					5.00%