Derivative and Hedging Financial Instruments - Summary of Volumes and Average Contract Prices of Contracts (Detail)	Sep. 30, 2016 MMBTU $ / MMBTU	Dec. 23, 2014 $ / MMBTU	Aug. 20, 2014 $ / MMBTU
Derivative [Line Items]
Commodity contracts, Basis Swap Fixed Price/MMBtu			(4.11)
Morgan Stanley Fixed III [Member]
Derivative [Line Items]
Volumes (MMBtu) | MMBTU	3,302,880
Commodity contracts, Basis Swap Fixed Price/MMBtu		(1.12)
Morgan Stanley Fixed III [Member] | 2016 [Member]
Derivative [Line Items]
Volumes (MMBtu) | MMBTU	574,741
Commodity contracts, Basis Swap Fixed Price/MMBtu	(1.12)
Morgan Stanley Fixed III [Member] | 2017 [Member]
Derivative [Line Items]
Volumes (MMBtu) | MMBTU	1,518,648
Commodity contracts, Basis Swap Fixed Price/MMBtu	(1.12)
Morgan Stanley Fixed III [Member] | 2018 [Member]
Derivative [Line Items]
Volumes (MMBtu) | MMBTU	1,209,491
Commodity contracts, Basis Swap Fixed Price/MMBtu	(1.12)
Morgan Stanley Restrike [Member]
Derivative [Line Items]
Volumes (MMBtu) | MMBTU	7,041,833
Commodity contracts, Basis Swap Fixed Price/MMBtu			(3.92)
Morgan Stanley Restrike [Member] | 2016 [Member]
Derivative [Line Items]
Volumes (MMBtu) | MMBTU	1,251,001
Commodity contracts, Weighted- Average Fixed Price/MMBtu	3.27
Morgan Stanley Restrike [Member] | 2017 [Member]
Derivative [Line Items]
Volumes (MMBtu) | MMBTU	3,248,187
Commodity contracts, Weighted- Average Fixed Price/MMBtu	3.27
Morgan Stanley Restrike [Member] | 2018 [Member]
Derivative [Line Items]
Volumes (MMBtu) | MMBTU	2,542,645
Commodity contracts, Weighted- Average Fixed Price/MMBtu	3.27