Income per Common Share (Summary Of Earnings Per Common Share) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Basic earnings per share:
Net income attributable to MPC	$ 626	$ 168	$ 593	$ 725	$ 755	$ 1,224	$ 814	$ 596	$ 2,112	$ 3,389	$ 2,389
Income allocated to participating securities									4	6	4
Income available to common stockholders - basic									2,108	3,383	2,385
Weighted average shares outstanding (in shares)									315	340	356
Basic earnings per share (in USD per share)	$ 2.09	$ 0.54	$ 1.84	$ 2.19	$ 2.26	$ 3.61	$ 2.39	$ 1.71	$ 6.69	$ 9.95	$ 6.70
Diluted earnings per share:
Net income attributable to MPC	626	168	593	725	755	1,224	814	596	2,112	3,389	2,389
Income allocated to participating securities									4	6	4
Income available to common stockholders - diluted									$ 2,108	$ 3,383	$ 2,385
Weighted average shares outstanding (in shares)									315	340	356
Effect of dilutive securities (in shares)									2	2	1
Weighted average common shares, including dilutive effect (in shares)									317	342	357
Diluted earnings per share (in USD per share)	$ 2.07	$ 0.54	$ 1.83	$ 2.17	$ 2.24	$ 3.59	$ 2.38	$ 1.70	$ 6.64	$ 9.89	$ 6.67