Derivative and Hedging Financial Instruments - Summary of Volumes and Average Contract Prices of Contracts (Detail)	Sep. 30, 2015 MMBTU $ / MMBTU	Dec. 23, 2014 $ / MMBTU	Aug. 20, 2014 $ / MMBTU	May. 21, 2014 $ / MMBTU
Morgan Stanley Fixed I [Member]
Derivative [Line Items]
Volumes (MMBtu)	8,789,797
Commodity contracts, Basis Swap Fixed Price/mmbtu | $ / MMBTU				(4.11)
Morgan Stanley Fixed I [Member] | 2015 [Member]
Derivative [Line Items]
Volumes (MMBtu)	893,151
Commodity contracts, Weighted- Average Fixed Price/mmbtu | $ / MMBTU	4.11
Morgan Stanley Fixed I [Member] | 2016 [Member]
Derivative [Line Items]
Volumes (MMBtu)	3,174,357
Commodity contracts, Weighted- Average Fixed Price/mmbtu | $ / MMBTU	3.27
Morgan Stanley Fixed I [Member] | 2017 [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,783,102
Commodity contracts, Weighted- Average Fixed Price/mmbtu | $ / MMBTU	3.27
Morgan Stanley Fixed I [Member] | 2018 [Member]
Derivative [Line Items]
Volumes (MMBtu)	1,939,187
Commodity contracts, Weighted- Average Fixed Price/mmbtu | $ / MMBTU	3.27
Morgan Stanley Fixed III [Member]
Derivative [Line Items]
Volumes (MMBtu)	5,301,585
Commodity contracts, Basis Swap Fixed Price/mmbtu | $ / MMBTU		(1.12)
Morgan Stanley Fixed III [Member] | 2015 [Member]
Derivative [Line Items]
Volumes (MMBtu)	553,830
Commodity contracts, Basis Swap Fixed Price/mmbtu | $ / MMBTU	(1.12)
Morgan Stanley Fixed III [Member] | 2016 [Member]
Derivative [Line Items]
Volumes (MMBtu)	1,878,290
Commodity contracts, Basis Swap Fixed Price/mmbtu | $ / MMBTU	(1.12)
Morgan Stanley Fixed III [Member] | 2017 [Member]
Derivative [Line Items]
Volumes (MMBtu)	1,539,998
Commodity contracts, Basis Swap Fixed Price/mmbtu | $ / MMBTU	(1.12)
Morgan Stanley Fixed III [Member] | 2018 [Member]
Derivative [Line Items]
Volumes (MMBtu)	1,329,467
Commodity contracts, Basis Swap Fixed Price/mmbtu | $ / MMBTU	(1.12)
Morgan Stanley Fixed II [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,750,113
Commodity contracts, Basis Swap Fixed Price/mmbtu | $ / MMBTU			(3.92)
Morgan Stanley Fixed II [Member] | 2015 [Member]
Derivative [Line Items]
Volumes (MMBtu)	382,817
Commodity contracts, Weighted- Average Fixed Price/mmbtu | $ / MMBTU	3.92
Morgan Stanley Fixed II [Member] | 2016 [Member]
Derivative [Line Items]
Volumes (MMBtu)	992,431
Commodity contracts, Weighted- Average Fixed Price/mmbtu | $ / MMBTU	3.27
Morgan Stanley Fixed II [Member] | 2017 [Member]
Derivative [Line Items]
Volumes (MMBtu)	771,407
Commodity contracts, Weighted- Average Fixed Price/mmbtu | $ / MMBTU	3.27
Morgan Stanley Fixed II [Member] | 2018 [Member]
Derivative [Line Items]
Volumes (MMBtu)	603,458
Commodity contracts, Weighted- Average Fixed Price/mmbtu | $ / MMBTU	3.27