Derivative Instruments - Derivative Instruments Related to Crude Oil Production/Processing Derivative Activities Table (Detail) (Crude Oil [Member])	Sep. 30, 2012
Contracted Commodities in 2012 [Member] | Three-Way Collars [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments	62
Contracted Commodities in 2012 [Member] | Three-Way Collars [Member] | Call Options Sold [Member]
Average price per barrel
Average ceiling price	122.30
Contracted Commodities in 2012 [Member] | Three-Way Collars [Member] | Put Options Purchased [Member]
Average price per barrel
Average floor price	101.22
Contracted Commodities in 2012 [Member] | Three-Way Collars [Member] | Put Options Sold [Member]
Average price per barrel
Average floor price	81.34
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	60
Average price per barrel
Average price per barrel	107.19
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	26
Contracted Commodities in 2013 [Member] | Three-Way Collars [Member] | Call Options Sold [Member]
Average price per barrel
Average ceiling price	125.15
Contracted Commodities in 2013 [Member] | Three-Way Collars [Member] | Put Options Purchased [Member]
Average price per barrel
Average floor price	105.00
Contracted Commodities in 2013 [Member] | Three-Way Collars [Member] | Put Options Sold [Member]
Average price per barrel
Average floor price	85.00
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	34
Average price per barrel
Average price per barrel	110.04