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Share Plans (Tables)
 9 Months Ended
Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Share-based Compensation Expense
Total share-based compensation expense for the three and nine months ended September 30, 2018 and 2017 was as follows:
 
Three months ended    
 
Nine months ended
In millions
September 30,
2018
September 30,
2017
 
September 30,
2018
September 30,
2017
Restricted stock units
$
2.4

$
2.9

 
$
6.7

$
14.4

Stock options
1.3

1.9

 
3.5

8.3

Performance share units
1.4

1.4

 
6.2

9.5

Total share-based compensation expense
$
5.1

$
6.2

 
$
16.4

$
32.2
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