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Share Plans (Tables)
 6 Months Ended
Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Share-based Compensation Expense
Total share-based compensation expense for the three and six months ended June 30, 2018 and 2017 was as follows:
 
Three months ended    
 
Six months ended
In millions
June 30,
2018
June 30,
2017
 
June 30,
2018
June 30,
2017
Restricted stock units
$
1.9

$
5.5

 
$
4.3

$
11.3

Stock options
1.0

2.0

 
2.2

6.5

Performance share units
2.4

2.1

 
4.8

8.2

Total share-based compensation expense
$
5.3

$
9.6

 
$
11.3

$
26.0
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions
We estimated the fair value of each stock option award issued in the annual share-based compensation grant using a Black-Scholes option pricing model, modified for dividends and using the following assumptions:
 
2018
Annual Grant
Risk-free interest rate
2.58
%
Expected dividend yield
1.56
%
Expected share price volatility
24.8
%
Expected term (years)
6.1