Deepwater Horizon Events - Liability Accrual, OA Liabilities, and OPA-Related Environmental Costs (Detail) (USD $)	Sep. 30, 2011	Sep. 30, 2011 Deepwater Horizon [Member] Oil Pollution Act of 1990 [Member]	Oct. 25, 2011 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]	Sep. 30, 2011 Deepwater Horizon [Member] BP Settlement Agreement [Member] BP indemnification liability [Member]
Loss Contingencies [Line Items]
Deepwater Horizon settlement and related costs	$ 4,017,000,000			$ 4,000,000,000
Loss contingency accrual at carrying value		0			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			7,000,000,000
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			$ 11,000,000,000
Allocable percentage share of gross OPA-related environmental costs		0.00%