Derivative Instruments - Crude Oil Production/Processing Derivative Activities Table (Detail) (Not Designated as Hedging Instrument [Member], 2013 Settlement [Member], Crude Oil [Member], Price Risk Derivative [Member])	9 Months Ended
	Sep. 30, 2013
Three-Way Collars [Member]
Derivative [Line Items]
Derivative nonmonetary notional amount per day	26
Three-Way Collars [Member] | Call Option [Member] | Short [Member]
Average price per barrel
Average ceiling price	125.15
Three-Way Collars [Member] | Put Option [Member] | Short [Member]
Average price per barrel
Average floor price	85.00
Three-Way Collars [Member] | Put Option [Member] | Long [Member]
Average price per barrel
Average floor price	105.00
Fixed-Price Contracts [Member]
Derivative [Line Items]
Derivative nonmonetary notional amount per day	167
Average price per barrel
Average price per barrel	102.74