Derivative Instruments - Natural Gas Production/Processing Derivative Activities Table (Detail) (2013 Settlement [Member], Natural Gas [Member])	Dec. 31, 2012
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, amount per day	0	[1]
Three-Way Collars [Member] | Offsetting Purchased and Sold Positions For Four Hundred Fifty Thousand Million British Thermal Units Per Day [Member] | Call Options Sold [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments, amount per day	450,000
Three-Way Collars [Member] | Offsetting Purchased and Sold Positions For Four Hundred Fifty Thousand Million British Thermal Units Per Day [Member] | Call Options Purchased [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments, amount per day	(450,000)
Fixed-Price Contracts [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments, amount per day	1,185,000
Average price per MMBtu	4.00

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