Derivative Instruments - Crude Oil Production/Processing Derivative Activities Table (Detail) (2013 Settlement [Member], Crude Oil [Member])	Dec. 31, 2012
Three-Way Collars [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments, amount per day	26,000
Three-Way Collars [Member] | Call Options Sold [Member]
Average price per barrel
Average ceiling price	125.15
Three-Way Collars [Member] | Put Options Purchased [Member]
Average price per barrel
Average floor price	105.00
Three-Way Collars [Member] | Put Options Sold [Member]
Average price per barrel
Average floor price	85.00
Fixed-Price Contracts [Member]
Derivative [Line Items]
Nonmonetary notional amount of price risk derivative instruments not designated as hedging instruments, amount per day	40,000
Average price per barrel
Average price per barrel	107.04