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Note 8 - Stock-based Compensation (Tables)
9 Months Ended
Sep. 30, 2020
Notes Tables  
Share-based Payment Arrangement, Option, Activity [Table Text Block]
      

Weighted-

 
      

Average

 
  

Stock Options

  

Exercise Price

 

Options outstanding at December 31, 2019

  276,266  $72.38 
Granted  60,065   105.79 
Exercised  (64,932)  57.24 
Forfeited  (37,842)  67.92 
Options outstanding at September 30, 2020  233,557  $85.89 
         
Exercisable at September 30, 2020  135,837  $98.38 
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
  

Nine Months Ended September 30,

  

2020

 

2019

Risk-free interest rate

 

0.50 - 1.47%

 

2.35 - 2.58%

Expected life (years)

 6.25 6.25

Expected volatility

 

42.40 - 43.83%

 

42.52 - 42.74%

Expected dividend yield

 

0%

 

0%

Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity [Table Text Block]
      

Weighted-

 
      

Average

 
      

Grant Date

 
  

Restricted

  

Fair Value

 
  

Stock

  

Per Share

 

Restricted stock at December 31, 2019

  314,973  $89.55 
Granted  108,179   117.76 
Restrictions lapsed  (100,596)  84.57 
Forfeited  (12,221)  92.61 
Restricted stock at September 30, 2020  310,335  $98.84 
Schedule of Nonvested Performance-based Units Activity [Table Text Block]
      

Weighted-

 
      

Average

 
      

Grant Date

 
  

Performance

  

Fair Value

 
  

Stock

  

Per Share

 

Performance stock at December 31, 2019

  9,383  $58.35 

Granted

  19,956   118.66 

Restrictions lapsed

  (7,113)  58.35 

Performance change

  -   - 

Forfeited

  (2,270)  58.35 

Performance stock at September 30, 2020

  19,956  $118.66 
Schedule of Share-based Payment Award, Awards Other Than Stock Options, Valuation Assumptions [Table Text Block]
  

Nine Months Ended September 30,

  

2020

 

Risk-free interest rate

 1.41% 

Expected life (years)

 2.88 

Expected volatility

 38.70% 

Expected dividend yield

 0% 
Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block]
  

Nine Months Ended September 30,

  

2020

 

2019

Risk-free interest rate

 

0.17 - 1.59%

 

2.33 - 2.35%

Expected life (months)

 6.00 6.00

Expected volatility

 

42.63 - 59.99%

 

37.36 - 53.57%

Expected dividend yield

 

0%

 

0%