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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2015
Summary of Activity Under Stock Option Plans
A summary of the activity under our stock option plans as of March 31, 2015 and changes during the three month period then ended, is presented below:

 
Options Outstanding
 
Weighted- Average Exercise Price Per Share
 
Weighted-Average Remaining Contractual Life in Years
 
Aggregate Intrinsic Value
Options outstanding at December 31, 2014
393,347

 
$
40.72

 
3.8
 
$
16,019,130

Options granted
—

 
—

 
 
 
 

Options exercised
(141,097
)
 
39.93

 
 
 
 

Options forfeited
(850
)
 
43.00

 
 
 
 

Options outstanding at March 31, 2015
251,400

 
41.06

 
3.7
 
10,344,139

Options exercisable at March 31, 2015
237,412

 
40.42

 
3.3
 
9,920,540

Options vested or expected to vest at March 31, 2015*
250,980

 
41.05

 
3.7
 
10,331,431

* In addition to the vested options, we expect a portion of the unvested options to vest at some point in the future. Options expected to vest are calculated by applying an estimated forfeiture rate to the unvested options.
Assumptions Used in Calculation of Fair Value
Below are the assumptions used in the Monte Carlo calculation:
 
March 31, 2015
 
March 31, 2014
Expected volatility
28.2%
 
33.7%
Expected term (in years)
3.0
 
3.0
Risk-free interest rate
0.96%
 
0.67%
Expected dividend yield
—
 
—
Performance-Based Restricted Stock [Member]  
Restricted Stock Activities
 
Performance-Based Restricted Stock Awards
Non-vested awards outstanding at December 31, 2014
92,437

Awards granted
50,798

Stock issued
(20,910
)
Awards forfeited
(6,110
)
Non-vested awards outstanding at March 31, 2015
116,215
Time Based Restricted Stock [Member]  
Restricted Stock Activities
 
Time-Based Restricted Stock Awards
Non-vested awards outstanding at December 31, 2014
238,386

Awards granted
62,135

Stock issued
(65,684
)
Awards forfeited
(2,960
)
Non-vested awards outstanding at March 31, 2015
231,877
Deferred Stock Units [Member]  
Restricted Stock Activities
 
Deferred Stock
Units
Awards outstanding at December 31, 2014
30,150

Awards granted
—

Stock issued
—

Awards outstanding at March 31, 2015
30,150