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Income per Common Share (Tables)
3 Months Ended
Mar. 31, 2018
Earnings Per Share [Abstract]  
Summary of Earnings Per Common Share
MPC grants certain incentive compensation awards to employees and non-employee directors that are considered to be participating securities. Due to the presence of participating securities, we have calculated our earnings per share using the two-class method.
 
Three Months Ended 
 March 31,
(In millions, except per share data)
2018
 
2017
Basic earnings per share:
 
 
 
Allocation of earnings:
 
 
 
Net income attributable to MPC
$
37

 
$
30

Income allocated to participating securities

 

Income available to common stockholders – basic
$
37

 
$
30

Weighted average common shares outstanding
476

 
525

Basic earnings per share
$
0.08

 
$
0.06

Diluted earnings per share:
 
 
 
Allocation of earnings:
 
 
 
Net income attributable to MPC
$
37

 
$
30

Income allocated to participating securities

 

Income available to common stockholders – diluted
$
37

 
$
30

Weighted average common shares outstanding
476

 
525

Effect of dilutive securities
4

 
5

Weighted average common shares, including dilutive effect
480

 
530

Diluted earnings per share
$
0.08

 
$
0.06

Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share
The following table summarizes the shares that were anti-dilutive and, therefore, were excluded from the diluted share calculation.
 
Three Months Ended 
 March 31,
(In millions)
2018
 
2017
Shares issued under stock-based compensation plans

 
2