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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Assumptions Used in Calculation of Fair Value
Below were the assumptions used in the Monte Carlo calculation:
 
 
September 30,
2017
 
September 30,
2016
Expected volatility
 
33.6%
 
29.6%
Expected term (in years)
 
3.0
 
3.0
Risk-free interest rate
 
1.38%
 
0.93%
Summary of Activity Under Stock Option Plans
A summary of the activity under our stock option plans during the three and nine month periods ended September 30, 2017 and changes during the 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, 2017
64,958

 
$
37.01

 
3.0
 
$
4,651,550

Options exercised
(1,400
)
 
$
24.20

 
 
 
 

Options forfeited
—

 
$
—

 
 
 
 

Options outstanding at September 30, 2017
63,558

 
$
37.29

 
2.8
 
$
6,100,702

Options exercisable at September 30, 2017
63,558

 
$
37.29

 
2.8
 
$
6,100,702

Options vested at September 30, 2017
63,558

 
$
37.29

 
2.8
 
$
6,100,702

 
Options Outstanding
 
Weighted- Average Exercise Price Per Share
Options outstanding at December 31, 2016
116,575

 
$
37.76

Options exercised
(53,017
)
 
$
36.33

Options forfeited
—

 
$
—

Options outstanding at September 30, 2017
63,558

 
$
37.29
Time Based Restricted Stock  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted Stock Activities
The following table summarizes the change in number of non-vested time-based restricted stock awards outstanding since December 31, 2016:
 
Time-Based Restricted Stock Awards
Non-vested awards outstanding at December 31, 2016
239,189

Awards granted
77,505

Stock issued
(132,373
)
Awards forfeited
(5,500
)
Non-vested awards outstanding at September 30, 2017
178,821
Performance-Based Restricted Stock  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted Stock Activities
The following table summarizes the change in number of non-vested performance-based restricted stock awards outstanding since December 31, 2016:
 
Performance-Based Restricted Stock Awards
Non-vested awards outstanding at December 31, 2016
151,769

Awards granted
56,147

Stock issued
(34,442
)
Awards forfeited
(4,047
)
Non-vested awards outstanding at September 30, 2017
169,427
Deferred Stock Units  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted Stock Activities
The following table summarizes the change in number of deferred stock units outstanding since December 31, 2016:
 
Deferred Stock Units
Awards outstanding at December 31, 2016
11,900

Awards granted
9,250

Stock issued
(11,900
)
Awards outstanding at September 30, 2017
9,250