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Stock Options (Tables)
12 Months Ended
Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]  
Weighted Average Assumptions Used in Estimating Fair Value of Stock Options

 

 

 

As of December 31,

 

 

 

2018

 

 

2017

 

 

2016

 

Valuation assumptions:

 

 

 

 

 

 

 

 

 

 

 

 

Expected dividend yield

 

 

0

%

 

 

0

%

 

 

0

%

Expected volatility

 

 

37.60

%

 

 

39.37

%

 

 

41.82

%

Expected term (years)

 

 

5.86

 

 

 

6.06

 

 

 

6.07

 

Risk-free interest rate

 

 

2.6

%

 

 

1.9

%

 

 

1.6

%

Summary of Outstanding Stock Option Activity

Stock option activity during the periods indicated is as follows:

 

 

 

Number

of shares

 

 

Weighted

average

exercise

price

 

 

Weighted

average

remaining contractual term

 

Balance at December 31, 2015

 

 

756,692

 

 

$

2.10

 

 

 

7.6

 

Granted

 

 

359,319

 

 

 

2.60

 

 

 

9.4

 

Exercised

 

 

(58,155

)

 

 

2.36

 

 

 

3.5

 

Expired/Forfeited

 

 

(17,469

)

 

 

2.13

 

 

 

2.6

 

Balance at December 31, 2016

 

 

1,040,387

 

 

$

2.25

 

 

 

7.8

 

Granted

 

 

471,144

 

 

 

14.72

 

 

 

9.1

 

Exercised

 

 

(284,455

)

 

 

2.07

 

 

 

5.2

 

Expired/Forfeited

 

 

(23,449

)

 

 

8.22

 

 

 

 

Balance at December 31, 2017

 

 

1,203,627

 

 

$

7.06

 

 

 

8.1

 

Granted

 

 

1,009,134

 

 

 

9.94

 

 

 

6.8

 

Exercised

 

 

(341,456

)

 

 

2.24

 

 

 

1.3

 

Expired/Forfeited

 

 

(464,256

)

 

 

11.81

 

 

 

 

Balance at December 31, 2018

 

 

1,407,049

 

 

$

8.73

 

 

 

8.1

 

Vested and exercisable at December 31, 2018

 

 

504,287

 

 

$

6.28

 

 

 

7.0

 

Vested and expected to vest at December 31, 2018

 

 

1,407,049

 

 

$

8.73

 

 

 

8.1