Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Contract Y	Dec. 31, 2011	Dec. 31, 2010
Commitments And Contingencies Disclosure [Line Items]
Interest accrues on the alleged damages	12.00%
Estimated damage claimed by plaintiff	$ 140
Rent expense under operating leases	13.8	18.6	18.5
Number of drilling rigs used under drilling contracts	3
Maximum period of drilling contracts in years	3
Maximum penalties on early termination of drilling contracts	20.6
Minimum remaining terms of leases on undeveloped acreage, in years	3 years
Maximum remaining terms of leases on undeveloped acreage, in years	5 years
2013 [Member]
Commitments And Contingencies Disclosure [Line Items]
Delivery commitments contingent upon pipeline construction or modification	12,329
Drilling contracts future minimum commitments	22.1
Amount of agreed upon payments for hydraulic fracturing services	24.0
Amount Of Agreement To Purchase Seismic Data	5.3
2013 [Member] | Appalachia [Member]
Commitments And Contingencies Disclosure [Line Items]
Delivery commitments contingent upon pipeline construction or modification	15,123
2014 [Member]
Commitments And Contingencies Disclosure [Line Items]
Delivery commitments contingent upon pipeline construction or modification	164,918
Drilling contracts future minimum commitments	10.8
Amount Of Agreement To Purchase Seismic Data	3.9
2014 [Member] | Appalachia [Member]
Commitments And Contingencies Disclosure [Line Items]
Delivery commitments contingent upon pipeline construction or modification	90,000
2015 [Member]
Commitments And Contingencies Disclosure [Line Items]
Delivery commitments contingent upon pipeline construction or modification	254,836
Drilling contracts future minimum commitments	$ 6.7
2015 [Member] | Appalachia [Member]
Commitments And Contingencies Disclosure [Line Items]
Delivery commitments contingent upon pipeline construction or modification	162,658
Thereafter [Member]
Commitments And Contingencies Disclosure [Line Items]
Delivery commitments contingent upon pipeline construction or modification	435,000
Thereafter [Member] | Appalachia [Member]
Commitments And Contingencies Disclosure [Line Items]
Delivery commitments contingent upon pipeline construction or modification	210,000