Derivative and Hedging Financial Instruments - Summary of Volumes and Average Contract Prices of Contracts (Detail)	Dec. 31, 2014 MMBTU
BP Hedge [Member]
Derivative [Line Items]
Volumes (MMBtu)	345,523
BP Hedge [Member] | 2015 [Member]
Derivative [Line Items]
Volumes (MMBtu)	345,523
Commodity contracts, Weighted- Average Floor Price/mmbtu	4.00
Commodity contracts, Weighted- Average Ceiling Price/mmbtu	4.28
Morgan Stanley Fixed I [Member]
Derivative [Line Items]
Volumes (MMBtu)	11,071,493
Morgan Stanley Fixed I [Member] | 2015 [Member]
Derivative [Line Items]
Volumes (MMBtu)	3,578,155
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.11
Morgan Stanley Fixed I [Member] | 2016 [Member]
Derivative [Line Items]
Volumes (MMBtu)	3,002,489
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.06
Morgan Stanley Fixed I [Member] | 2017 [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,495,153
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.16
Morgan Stanley Fixed I [Member] | 2018 [Member]
Derivative [Line Items]
Volumes (MMBtu)	1,995,696
Commodity contracts, Weighted- Average Fixed Price/mmbtu	4.29
Morgan Stanley Fixed II [Member]
Derivative [Line Items]
Volumes (MMBtu)	4,405,924
Morgan Stanley Fixed II [Member] | 2015 [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,038,628
Commodity contracts, Weighted- Average Fixed Price/mmbtu	3.92
Morgan Stanley Fixed II [Member] | 2016 [Member]
Derivative [Line Items]
Volumes (MMBtu)	1,067,313
Commodity contracts, Weighted- Average Fixed Price/mmbtu	3.92
Morgan Stanley Fixed II [Member] | 2017 [Member]
Derivative [Line Items]
Volumes (MMBtu)	753,034
Commodity contracts, Weighted- Average Fixed Price/mmbtu	3.92
Morgan Stanley Fixed II [Member] | 2018 [Member]
Derivative [Line Items]
Volumes (MMBtu)	546,949
Commodity contracts, Weighted- Average Fixed Price/mmbtu	3.92
Morgan Stanley Fixed III [Member]
Derivative [Line Items]
Volumes (MMBtu)	7,301,209
Morgan Stanley Fixed III [Member] | 2015 [Member]
Derivative [Line Items]
Volumes (MMBtu)	2,553,454
Commodity contracts, Basis Swap Fixed Price/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	(1.12)
Morgan Stanley Fixed III [Member] | 2017 [Member]
Derivative [Line Items]
Volumes (MMBtu)	1,539,998
Commodity contracts, Basis Swap Fixed Price/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	(1.12)