Derivative and Hedging Financial Instruments - Summary of Volumes and Average Contract Prices of Contracts (Detail)	9 Months Ended
	Sep. 30, 2014 MMBTU
Derivative [Line Items]
Volumes (MMBtu)	836,633
Gas [Member] | Commodity Contract 1 [Member]
Derivative [Line Items]
Contract period	2014
Volumes (MMBtu)	371,808
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
Morgan Stanley Fixed II [Member]
Derivative [Line Items]
Volumes (MMBtu)	10,445,238
Morgan Stanley Fixed II [Member] | Commodity Contract 1 [Member]
Derivative [Line Items]
Volumes (MMBtu)	586,700
Commodity contracts, Weighted- Average Fixed Price/mmbtu	3.92
Morgan Stanley Fixed II [Member] | Commodity Contract 2 [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,249,200
Commodity contracts, Weighted- Average Fixed Price/mmbtu	3.92
Morgan Stanley Fixed II [Member] | Commodity Contract 3 [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,887,048
Commodity contracts, Weighted- Average Fixed Price/mmbtu	3.92
Morgan Stanley Fixed II [Member] | Commodity Contract Four [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,528,996
Commodity contracts, Weighted- Average Fixed Price/mmbtu	3.92
Morgan Stanley Fixed II [Member] | Commodity Contract Five [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,193,294
Commodity contracts, Weighted- Average Fixed Price/mmbtu	3.92
Morgan Stanley Fixed I [Member]
Derivative [Line Items]
Volumes (MMBtu)	12,334,528
Morgan Stanley Fixed I [Member] | Commodity Contract 1 [Member]
Derivative [Line Items]
Volumes (MMBtu)	971,987
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.38
Morgan Stanley Fixed I [Member] | Commodity Contract 2 [Member]
Derivative [Line Items]
Volumes (MMBtu)	3,581,893
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.11
Morgan Stanley Fixed I [Member] | Commodity Contract 3 [Member]
Derivative [Line Items]
Volumes (MMBtu)	3,058,358
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.06
Morgan Stanley Fixed I [Member] | Commodity Contract Four [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,528,996
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.16
Morgan Stanley Fixed I [Member] | Commodity Contract Five [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,193,294
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.29