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Share Plans (Tables)
 6 Months Ended
Jun. 30, 2017
Share-based Compensation Expense
Total share-based compensation expense for the three and six months ended June 30, 2017 and 2016 was as follows:
 
Three months ended    
 
Six months ended
In millions
June 30,
2017
June 30,
2016
 
June 30,
2017
June 30,
2016
Restricted stock units
$
5.5

$
3.6

 
$
11.3

$
10.6

Stock options
2.0

1.8

 
6.5

7.2

Performance share units
2.1

0.8

 
8.2

4.5

Total share-based compensation expense
$
9.6

$
6.2

 
$
26.0

$
22.3
Assumptions Used in Calculating Estimated Fair Value of Stock Option Award
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:
 
2017
Annual Grant
Risk-free interest rate
1.61
%
Expected dividend yield
2.38
%
Expected share price volatility
26.9
%
Expected term (years)
6.3