Derivative and Hedging Financial Instruments - Summary of Volumes and Average Contract Prices of Contracts (Detail)	6 Months Ended
	Jun. 30, 2014 MMBTU
Derivative [Line Items]
Volumes (MMBtu)	1,511,245
Gas [Member] | Commodity Contract 1 [Member]
Derivative [Line Items]
Contract period	2014
Volumes (MMBtu)	1,046,420
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	2015
Volumes (MMBtu)	464,825
Commodity contracts, Weighted- Average Floor Price/mmbtu	4.00
Commodity contracts, Weighted- Average Ceiling Price/mmbtu	4.28
Gas Hedges [Member]
Derivative [Line Items]
Volumes (MMBtu)	13,932,171
Gas Hedges [Member] | Commodity Contract 1 [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,860,678
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.38
Gas Hedges [Member] | Commodity Contract 2 [Member]
Derivative [Line Items]
Volumes (MMBtu)	3,578,155
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.11
Gas Hedges [Member] | Commodity Contract 3 [Member]
Derivative [Line Items]
Volumes (MMBtu)	3,002,489
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.06
Gas Hedges [Member] | Commodity Contract Four [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,495,153
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.16
Gas Hedges [Member] | Commodity Contract Five [Member]
Derivative [Line Items]
Volumes (MMBtu)	1,995,696
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.29