Supplemental Oil and Gas Disclosures - Principal Sources of Change in Discounted Future Net Cash Flows (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Changes In Standardized Measure Of Discounted Future Net Cash Flows [Line Items]
Sales, net of production costs	$ (12,589)	$ (13,152)	$ (9,152)
Net change in prices and production costs	(1,941)	12,167	13,006
Discoveries and improved recovery, net of related costs	6,742	6,751	5,147
Change in future development costs	(935)	(2,250)	(1,637)
Previously estimated development costs incurred during the period	4,359	2,479	1,355
Revision of quantities	(4,065)	(1,475)	(1,905)
Purchases of minerals in-place	1,181	2,139	7,794
Accretion of discount	5,234	4,161	2,439
Change in income taxes	2,711	(4,303)	(4,535)
Sales of properties	(3)	(1,285)	(3)
Change in production rates and other	(2,088)	273	182
Change in the discounted future net cash flows, total	$ (1,394)	$ 5,505	$ 12,691