Derivative Instruments - Derivative Instruments Related to Natural Gas Production/Processing Derivative Activities Table (Detail) (Natural Gas [Member])	Sep. 30, 2012
Contracted Commodities in 2012 [Member] | Three-Way Collars [Member] | Offsetting Purchased and Sold Positions For Five Hundred Thousand Million British Thermal Units Per Day [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	0	[1]
Contracted Commodities in 2012 [Member] | Fixed-Price Contracts [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	1,000,000
Average price per MMBtu	4.69
Contracted Commodities in 2013 [Member] | Three-Way Collars [Member] | Offsetting Purchased and Sold Positions For Four Hundred Fifty Thousand Million British Thermal Units Per Day [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	0	[1]
Contracted Commodities in 2013 [Member] | Fixed-Price Contracts [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	900,000
Average price per MMBtu	4.00

[1] The Company has entered into offsetting purchased and sold natural-gas three-way collars of 500,000 MMBtu/d and 450,000 MMBtu/d for 2012 and 2013, respectively.