Acquisitions - Schedule of Acquisition Related Pro Forma Financial Information (Detail) (Galveston Bay Refinery and Related Assets [Member], USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Galveston Bay Refinery and Related Assets [Member]
Business Acquisition [Line Items]
Sales and other operating revenues (including consumer excise taxes)	$ 26,888	$ 77,224	$ 77,714
Net income attributable to MPC	$ 1,530	$ 1,541	$ 2,798
Net income attributable to MPC per share - basic (in USD per share)	$ 4.53	$ 4.80	$ 8.18
Net income attributable to MPC per share - diluted (in USD per share)	$ 4.50	$ 4.77	$ 8.13