Derivative Instruments - Derivative Instruments Related to Natural Gas Production/Processing Derivative Activities Table (Detail) (Natural Gas [Member])	Dec. 31, 2011
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
Average price per MMBtu
Average price per MMBtu	4.69
Contracted Commodities in 2013 [Member] | Three-Way Collars [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	450
Contracted Commodities in 2013 [Member] | Three-Way Collars [Member] | Call Options Sold [Member]
Average price per MMBtu
Average ceiling price	6.57
Contracted Commodities in 2013 [Member] | Three-Way Collars [Member] | Put Options Purchased [Member]
Average price per MMBtu
Average floor price	5.00
Contracted Commodities in 2013 [Member] | Three-Way Collars [Member] | Put Options Sold [Member]
Average price per MMBtu
Average floor price	4.00

[1] Includes the effects of offsetting purchased and sold natural-gas three-way collars of 500,000 MMBtu/d.