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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-based compensation
Stock-based compensation for the periods ended June 30 is as follows:
(in millions)
Three Months
 
Six Months
 
2017
 
2016
 
2017
 
2016
Stock option expense
$
—

 
$
2

 
$
2

 
$
4

Restricted stock and unit awards expense
21

 
18

 
39

 
30

Total stock-based compensation expense 
$
21

 
$
20

 
$
41

 
$
34