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Earnings Per Share
 12 Months Ended
Dec. 31, 2020
Earnings Per Share [Abstract]  
Earnings Per Share 8. Earnings Per Share

The following table provides a reconciliation between basic and diluted earnings per share for the years ended December 31:

Millions, Except Per Share Amounts

2020

2019

2018

Net income

$

5,349 

$

5,919 

$

5,966 

Weighted-average number of shares outstanding:

Basic

677.3 

703.5 

750.9 

Dilutive effect of stock options

0.8 

1.2 

1.9 

Dilutive effect of retention shares and units

1.0 

1.4 

1.5 

Diluted

679.1 

706.1 

754.3 

Earnings per share – basic

$

7.90 

$

8.41 

$

7.95 

Earnings per share – diluted

$

7.88 

$

8.38 

$

7.91 

Common stock options totaling 0.3 million, 0.5 million, and 0.3 million for 2020, 2019, and 2018, respectively, were excluded from the computation of diluted earnings per share because the exercise prices of these options exceeded the average market price of our common stock for the respective periods, and the effect of their inclusion would be anti-dilutive.