Deepwater Horizon Events - Liability Accrual, OA Liabilities, and OPA-Related Environmental Costs (Detail) (USD $)						3 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Deepwater Horizon [Member] BP indemnification liability [Member]	Dec. 31, 2011 Deepwater Horizon [Member] Oil Pollution Act of 1990 [Member]	Feb. 07, 2012 Deepwater Horizon [Member] Oil Pollution Act of 1990 [Member] BP Exploration and Production Inc. [Member]	Sep. 30, 2011 Deepwater Horizon [Member] BP Settlement Agreement [Member]
Loss Contingencies [Line Items]
Deepwater Horizon settlement and related costs						$ 4,000,000,000
Loss contingency accrual at carrying value	342,000,000	114,000,000	0
Gross OPA-related environmental costs, minimum, excluding amounts BP has already funded, amounts that cannot reasonably be estimated by BP, and non-OPA-related fines and penalties					6,000,000,000
Gross OPA-related environmental costs, maximum, excluding amounts BP has already funded, amounts that cannot reasonably be estimated by BP, and non-OPA-related fines and penalties					10,000,000,000
Allocable percentage share of gross OPA-related environmental costs				0.00%
Accrual for environmental loss contingencies	$ 92,000,000	$ 96,000,000		$ 0