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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Stock-Based Compensation
Stock-based compensation for the periods ended June 30 is as follows:
(in millions)
Three Months
 
Six Months
 
2018
 
2017
 
2018
 
2017
Stock option expense
$
2

 
$
—

 
$
2

 
$
2

Restricted stock and unit awards expense
32

 
21

 
44

 
39

Total stock-based compensation expense 
$
34

 
$
21

 
$
46

 
$
41