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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-based compensation
Stock-based compensation for the three months ended March 31 is as follows:
(in millions)
2017
 
2016
Stock option expense
$
1

 
$
2

Restricted stock and unit awards expense
18

 
12

Total stock-based compensation expense 
$
19

 
$
14