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Stock Compensation Plans Employee Stock Compensation Plans (Tables)
 3 Months Ended
Mar. 31, 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 March 31
2018
 
2017
Restricted stock units
$
13

 
$
18

Employee stock purchase plan
1

 
1

Stock-based compensation expense
14

 
19

Tax benefit
(3)

 
(6)

Total stock-based compensation expense (net of tax)
$
11

 
$
13
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
During the three months ended March 31, 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 three months ended March 31, 2018. During the three months ended March 31, 2017 the Company did not grant any stock options.

 
Three months ended March 31
2018
Dividend yield
$
—

Risk-free interest rate
2.47
%
Expected volatility
34.81
%
Expected holding period (years)
3.8