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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2015
Summary of Activity Under Stock Option Plans
A summary of the activity under our stock option plans as of September 30, 2015 and changes during the quarter and nine months 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 June 30, 2015
224,238

 
$
40.49

 
3.5
 
$
5,752,806

Options exercised
(600
)
 
$
40.70

 
 
 
 

Options forfeited
—

 
$
—

 
 
 
 

Options outstanding at September 30, 2015
223,638

 
$
40.48

 
3.3
 
2,985,568

Options exercisable at September 30, 2015
222,827

 
$
39.98

 
3.0
 
3,088,366

Options vested or expected to vest at September 30, 2015*
223,614

 
$
40.47

 
3.3
 
2,988,652

* 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.
 
 
 
Options
Outstanding
 
Weighted-
Average
Exercise Price
Per Share
Options outstanding at December 31, 2014
393,347

 
$
40.72

Options exercised
(168,259
)
 
$
40.91

Options canceled
(1,450
)
 
$
39.62

Options outstanding at September 30, 2015
223,638

 
 
Assumptions Used in Calculation of Fair Value
Below are the assumptions used in the Monte Carlo calculation:
 
September 30, 2015
 
September 30, 2014
Expected volatility
28.2%
 
33.7%
Expected term (in years)
3.0
 
3.0
Risk-free interest rate
0.96%
 
0.67%
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
(10,690
)
Non-vested awards outstanding at September 30, 2015
111,635
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
70,060

Stock issued
(72,986
)
Awards forfeited
(7,168
)
Non-vested awards outstanding at September 30, 2015
228,292
Deferred Stock Units [Member]  
Restricted Stock Activities
 
Deferred Stock
Units
Awards outstanding at December 31, 2014
30,150

Awards granted
9,100

Stock issued
(15,400
)
Awards outstanding at September 30, 2015
23,850