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Stock Compensation Plans Employee Stock Compensation Plans (Tables)
 6 Months Ended
Jun. 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 June 30
 
Six months ended June 30
2018
 
2017
 
2018
 
2017
Restricted stock units
$
24

 
$
21

 
$
37

 
$
39

Stock options
1

 
—

 
1

 
—

Employee stock purchase plan
1

 
1

 
2

 
2

Stock-based compensation expense
26

 
22

 
40

 
41

Tax benefit
(4)

 
(6)

 
(7
)
 
(12
)
Total stock-based compensation expense (net of tax)
$
22

 
$
16

 
$
33

 
$
29
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
During the six months ended June 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 six months ended June 30, 2018. During the six months ended June 30, 2017 the Company did not grant any stock options.

 
Six months ended June 30, 2018
Dividend yield
$
—

Risk-free interest rate
2.5
%
Expected volatility
34.8
%
Expected holding period (years)
3.8