Dispositions - Additional Information (Detail) (USD $)	9 Months Ended	3 Months Ended			9 Months Ended		3 Months Ended	9 Months Ended		6 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 West Texas	Sep. 30, 2013 North Texas	Jun. 30, 2013 Delaware and Permian Basin	Sep. 30, 2013 Delaware and Permian Basin	Feb. 26, 2013 Delaware and Permian Basin	Jun. 30, 2013 Pennsylvania And West Texas	Sep. 30, 2013 Pennsylvania And West Texas	Sep. 30, 2012 Pennsylvania And West Texas	Jun. 30, 2012 Marcellus Shale	Sep. 30, 2012 Marcellus Shale	Mar. 31, 2012 East Texas	Sep. 30, 2012 East Texas
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 on sale of equity method investment in a drilling company	4,400,000
Gain (loss) on disposition of oil and gas property		1,700,000			83,500,000			4,000,000	746,000		(2,500,000)		(10,900,000)
Gain (Loss) on disposition of other assets			(253,000)
Selling expenses for property sale				4,200,000
Proceeds from sale of oil and gas		2,600,000					14,200,000	10,000,000	13,900,000	2,500,000		8,600,000
Proceeds from Sale of Other Assets			5,300,000
Proceeds from sale of equity method investment in a drilling company	$ 7,000,000
Sale of oil and gas properties												75.00%
Overriding royalty in the prospect minimum									1.00%			2.50%
Overriding royalty in the prospect maximum									5.00%			5.00%