Risk Management and Trading Activities - Corporate Risk Management Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Mar. 31, 2013 bbl	Dec. 31, 2012 bbl	Sep. 30, 2013 Interest rate swap [Member]	Dec. 31, 2012 Interest rate swap [Member]	Sep. 30, 2013 New Brent Crude Hedges [Member]	Sep. 30, 2012 New Brent Crude Hedges [Member]	Sep. 30, 2013 New Brent Crude Hedges [Member]	Sep. 30, 2012 New Brent Crude Hedges [Member]	Sep. 30, 2013 Corporate Risk Management [Member] Foreign exchange [Member]	Sep. 30, 2012 Corporate Risk Management [Member] Foreign exchange [Member]	Sep. 30, 2013 Corporate Risk Management [Member] Foreign exchange [Member]	Sep. 30, 2012 Corporate Risk Management [Member] Foreign exchange [Member]	Sep. 30, 2013 Corporate Risk Management [Member] Closed & New Brent Crude Hedges [Member]	Sep. 30, 2013 Corporate Risk Management [Member] New Brent Crude Hedges [Member]	Sep. 30, 2012 Corporate Risk Management [Member] New Brent Crude Hedges [Member]	Sep. 30, 2013 Corporate Risk Management [Member] New Brent Crude Hedges [Member]	Sep. 30, 2012 Corporate Risk Management [Member] New Brent Crude Hedges [Member]	Sep. 30, 2013 Sales and other operating revenues [Member]	Sep. 30, 2012 Sales and other operating revenues [Member]	Sep. 30, 2013 Sales and other operating revenues [Member]	Sep. 30, 2012 Sales and other operating revenues [Member]
Derivative [Line Items]
Total number of barrels per day hedged based on new Brent crude oil cash flow hedges	90,000	120,000
Average price for hedges (per barrel)	109.70	107.70
Number of barrels per day hedged crude oil that realized hedge losses		24,000
Realized gains (losses) from E&P hedging activities due to the increase (decrease) in sales and other operating revenue																		$ 2	$ (148)	$ 36	$ (533)
Realized gains (losses) from E&P hedging activities net of taxes																		1	(94)	23	(334)
After-tax deferred gains in Accumulated other comprehensive income (loss)													1
Gain (loss) on ineffectiveness reflected in earnings														(17)	(2)	1	(10)
Derivative Notional Amount			865	880
Change in the carrying value of the hedged fixed-rate debt - increase/(decrease)					1	7	(27)	17
Change in the fair value of interest rate swaps - increase/(decrease)					1	7	(27)	17
Net realized and unrealized pre-tax gains (losses) on foreign exchange contracts									$ 4	$ 31	$ (36)	$ 35