Derivative and Hedging Financial Instruments - Summary of Volumes and Average Contract Prices of Contracts (Detail)	9 Months Ended
	Sep. 30, 2013 MMBTU
Derivative [Line Items]
Volumes (MMBtu)	2,619,134
Gas [Member] | Commodity Contract 1 [Member]
Derivative [Line Items]
Contract period	2013
Volumes (MMBtu)	504,061
Commodity contracts, Weighted- Average Floor Price/mmbtu	4.00
Commodity contracts, Weighted- Average Ceiling Price/mmbtu	4.28
Gas [Member] | Commodity Contract 2 [Member]
Derivative [Line Items]
Contract period	2014
Volumes (MMBtu)	1,650,248
Commodity contracts, Weighted- Average Floor Price/mmbtu	4.00
Commodity contracts, Weighted- Average Ceiling Price/mmbtu	4.28
Gas [Member] | Commodity Contract 3 [Member]
Derivative [Line Items]
Contract period	2015
Volumes (MMBtu)	464,825
Commodity contracts, Weighted- Average Floor Price/mmbtu	4.00
Commodity contracts, Weighted- Average Ceiling Price/mmbtu	4.28