Acquisitions and Divestitures - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended		9 Months Ended						1 Months Ended		1 Months Ended	3 Months Ended			3 Months Ended
	Sep. 30, 2012 ft3	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011 ft3	Sep. 30, 2012 Eagle Ford Well	Dec. 31, 2011 Eagle Ford acre Well	Dec. 31, 2011 Eagle Ford Maximum Well	Dec. 31, 2011 Eagle Ford Minimum	Dec. 31, 2011 Eagle Ford Partner Well	Jan. 31, 2012 Counties of Butler and Armstrong in Pennsylvania	Sep. 30, 2012 Counties of Butler and Armstrong in Pennsylvania	Jun. 30, 2012 Counties of Butler and Armstrong in Pennsylvania	Oct. 31, 2012 Subsequent event Eagle Ford acre	Dec. 31, 2012 Subsequent event Eagle Ford Shale, Gonzales [Member]	Dec. 31, 2012 Subsequent event Eagle Ford Shale, Lavaca [Member]	Oct. 31, 2012 Subsequent event Post-participation [Member] Eagle Ford acre	Oct. 31, 2012 Subsequent event Pre-participation [Member] Eagle Ford acre
Business Acquisition [Line Items]
Working Interest, Non-Op Owners																17.00%	46.00%
AMI Acreage (in acres)								13,500
Working interest										57.00%
Gas and Oil Acreage, Acquired, Net																		3,000	4,100
Asset Acquisition, Purchase Price															$ 10,000,000
Gas and Oil Acreage, Undeveloped, Net															30,000
Asset Impairment Charges		700,000	0	29,316,000	71,071,000									28,600,000
Number of wells to be drilled									6		3
Number of wells drilled							6.2	7
Proceeds from the sale of oil and gas property, net				93,276,000	31,077,000							1,000,000	100,000,000
Consent Property Value													4,000,000
Proceeds from Sale of Property, Plant and Equipment, Net													92,200,000
Gain (Loss) on Sale of Property Plant Equipment				2,407,000	223,000								1,700,000
June Daily Gas Production (in ft3)	20,000,000
Percentage Of Gas Production	100.00%
Proved Developed Reserves (Volume ft3)						106,000,000,000
Percent Gas Reserves						96.00%
Gain on sale of undeveloped acres												$ 600,000