Derivative Instruments - Summary of Outstanding Contracts with Respect to Natural Gas (Detail) (Natural Gas [Member])	6 Months Ended
	Jun. 30, 2013 MMBTU
July 2013 - December 2013
Derivative [Line Items]
Natural Gas Production Derivative Volume, MMBtus	46,000,000
Swaps Weighted Average Price	3.78
January 2014 - March 2014
Derivative [Line Items]
Natural Gas Production Derivative Volume, MMBtus	14,400,000
Swaps Weighted Average Price	4.30
April 2014 - December 2014
Derivative [Line Items]
Natural Gas Production Derivative Volume, MMBtus	30,250,000
Swaps Weighted Average Price	4.14