Earnings Per Share - Summary of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic earnings per share:
Net income attributable to MPC	$ 1,106	$ 1,055	$ 1,099	$ 1,092
Income allocated to participating securities	0	1	1	1
Income available to common stockholders – basic	$ 1,106	$ 1,054	$ 1,098	$ 1,091
Weighted average common shares outstanding (in shares)	662	459	667	467
Basic earnings per share	$ 1.67	$ 2.30	$ 1.65	$ 2.34
Diluted earnings per share:
Net income attributable to MPC	$ 1,106	$ 1,055	$ 1,099	$ 1,092
Income allocated to participating securities	0	1	1	1
Income available to common stockholders – diluted	$ 1,106	$ 1,054	$ 1,098	$ 1,091
Weighted average common shares outstanding (in shares)	662	459	667	467
Effect of dilutive securities (in shares)	4	5	5	5
Weighted average common shares, including dilutive effect (in shares)	666	464	672	472
Diluted earnings per share	$ 1.66	$ 2.27	$ 1.63	$ 2.31