Derivative Instruments Related to Crude Oil Production and Processing Activities Table (Detail) (Three-Way Collars [Member], Crude Oil [Member])	Sep. 30, 2011
Contracted Commodities in 2011 [Member] | Call Options Sold [Member]
Average price per barrel
Average ceiling price	99.95
Contracted Commodities in 2011 [Member] | Put Options Purchased [Member]
Average price per barrel
Average floor price	79.29
Contracted Commodities in 2011 [Member] | Put Options Sold [Member]
Average price per barrel
Average floor price	64.29
Contracted Commodities in 2011 [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	126
Contracted Commodities in 2012 [Member] | Call Options Sold [Member]
Average price per barrel
Average ceiling price	92.50
Contracted Commodities in 2012 [Member] | Put Options Purchased [Member]
Average price per barrel
Average floor price	50.00
Contracted Commodities in 2012 [Member] | Put Options Sold [Member]
Average price per barrel
Average floor price	35.00
Contracted Commodities in 2012 [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	2