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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2018
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:
 
 
March 31,
2018
 
March 31,
2017
Expected volatility
 
34.8%
 
33.6%
Expected term (in years)
 
3.0
 
3.0
Risk-free interest rate
 
2.28%
 
1.38%
Summary of Activity Under Stock Option Plans
A summary of the activity under our stock option plans during the three months ended March 31, 2018 and changes during the three 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 December 31, 2017
33,283

 
$
36.40

 
2.2
 
$
4,177,655

Options exercised
(11,383
)
 
$
41.56

 
 
 
 
Options forfeited
—

 
$
—

 
 
 
 
Options outstanding at March 31, 2018
21,900

 
$
33.72

 
2.1
 
$
1,879,485

Options exercisable at March 31, 2018
21,900

 
$
33.72

 
2.1
 
$
1,879,485

Options vested at March 31, 2018
21,900

 
$
33.72

 
2.1
 
$
1,879,485
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 time-based restricted stock awards outstanding for the three months ended March 31, 2018:
 
Time-Based Restricted Stock Awards
Awards outstanding at December 31, 2017
173,331

Awards granted
41,100

Stock issued
(76,996
)
Awards forfeited
(1,990
)
Awards outstanding at March 31, 2018
135,445
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 performance-based restricted stock awards outstanding for the three months ended March 31, 2018:
 
Performance-Based Restricted Stock Awards
Awards outstanding at December 31, 2017
169,202

Awards granted
72,160

Stock issued
(81,230
)
Awards forfeited
(1,735
)
Awards outstanding at March 31, 2018
158,397
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 during the three months ended March 31, 2018:
 
Deferred Stock Units
Awards outstanding at December 31, 2017
9,250

Awards granted
—

Stock issued
—

Awards outstanding at March 31, 2018
9,250