Income Per Common Share - Summary of Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions |
3 Months Ended | |
|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
|
| Basic earnings per share: | ||
| Net income attributable to MPC | $ 37 | $ 30 |
| Income allocated to participating securities | 0 | 0 |
| Income available to common stockholders – basic | $ 37 | $ 30 |
| Weighted average common shares outstanding (in shares) | 476 | 525 |
| Basic (in USD per share) | $ 0.08 | $ 0.06 |
| Diluted earnings per share: | ||
| Net income attributable to MPC | $ 37 | $ 30 |
| Income allocated to participating securities | 0 | 0 |
| Income available to common stockholders – diluted | $ 37 | $ 30 |
| Weighted average common shares outstanding (in shares) | 476 | 525 |
| Effect of dilutive securities (in shares) | 4 | 5 |
| Weighted average common shares, including dilutive effect (in shares) | 480 | 530 |
| Diluted (in USD per share) | $ 0.08 | $ 0.06 |