Segment Information (Reconciliation Of Segment Income From Operations To Income Before Income Taxes) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Income before income taxes	$ 1,870		$ 1,744		$ 4,094		$ 3,899
Segment income from operations [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Income before income taxes	1,819		1,852		4,336		4,132
Corporate And Other Unallocated Items [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Income before income taxes	(74)	[1]	(93)	[1]	(245)	[1]	(229)	[1]
Minnesota Assets sale settlement gain [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Income before income taxes	183	[2]			183	[2]
Pension Settlement Expenses [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Income before income taxes	(33)	[3]			(116)	[3]
Net Interest And Other Financial Income (Costs) [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Income before income taxes	$ (25)		$ (15)		$ (64)		$ (4)

[1] Corporate and other unallocated items consists primarily of MPC's corporate administrative expenses, including allocations from the Marathon Oil Companies for periods prior to the Spinoff and costs related to certain non-operating assets.

[2] See Note 5.

[3] See Note 18.