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Stock Compensation Plans Employee Stock Compensation Plans (Tables)
 9 Months Ended
Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block]
Stock-based compensation expense for the following periods were:
In millions
Three months ended September 30
 
Nine months ended September 30
2018
 
2017
 
2018
 
2017
Restricted stock units
$
13

 
$
18

 
$
50

 
$
57

Stock options
1

 
—

 
2

 
—

Employee stock purchase plan
1

 
1

 
3

 
3

Stock-based compensation expense
15

 
19

 
55

 
60

Tax benefit
—

 
(9)

 
(7
)
 
(21
)
Total stock-based compensation expense (net of tax)
$
15

 
$
10

 
$
48

 
$
39
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
During the nine months ended September 30, 2018, the Company granted stock options and the weighted average fair value of option grants was estimated based on the below weighted average assumptions, which was $9.80 for the nine months ended September 30, 2018. During the nine months ended September 30, 2017 the Company did not grant any stock options.

 
Nine months ended September 30, 2018
Dividend yield
$
—

Risk-free interest rate
2.50
%
Expected volatility
34.88
%
Expected holding period (years)
3.8