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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of stock-based compensation
Stock-based compensation for the periods ended September 30 is as follows:
(in millions)
Three Months
 
Nine Months
 
2017
 
2016
 
2017
 
2016
Stock option expense
$
1

 
$
2

 
$
2

 
$
6

Restricted stock and unit awards expense
23

 
18

 
63

 
48

Total stock-based compensation expense 
$
24

 
$
20

 
$
65

 
$
54