Risk Management and Trading Activities - Changes in Physical Derivative and Financial Assets and Liabilities that are Measured at Fair Value Based on Level 3 Inputs (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fair Value Assets Measured On Recurring Basis [Line Items]
Level 3 Unrealized pre-tax gains (losses)	$ (78)	$ (52)
Sales and other operating revenues [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Level 3 Unrealized pre-tax gains (losses)	(44)
Cost of Products Sold [Member]
Fair Value Assets Measured On Recurring Basis [Line Items]
Level 3 Unrealized pre-tax gains (losses)	$ (34)