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Note 9 - Stock-based Compensation (Tables)
9 Months Ended
Sep. 30, 2021
Notes Tables  
Share-based Payment Arrangement, Option, Activity [Table Text Block]
      

Weighted-

 
      

Average

 
  

Stock Options

  

Exercise Price

 

Options outstanding at December 31, 2020

  229,531  $86.46 

Granted

  57,901   128.14 

Exercised

  (28,991)  58.78 

Forfeited

  (9,893)  100.58 

Options outstanding at September 30, 2021

  248,548  $98.84 
         

Exercisable at September 30, 2021

  116,088  $81.47 
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
  

Nine Months Ended September 30,

 
  

2021

  

2020

 

Risk-free interest rate

  

0.80 - 1.12%

   

0.50 - 1.47%

 

Expected life (years)

  6.25   6.25 

Expected volatility

  

45.28 - 45.53%

   

42.40 - 43.83%

 

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, 2020

  307,199  $98.87 

Granted

  180,798   132.84 

Restrictions lapsed

  (113,315)  91.10 

Forfeited

  (22,908)  120.01 

Restricted stock at September 30, 2021

  351,774  $117.51 
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, 2020

  19,956  $118.66 

Granted

  15,078   203.64 

Restrictions lapsed

  -   - 

Performance change

  (8,226)  192.95 

Forfeited

  (4,184)  99.59 

Performance stock at September 30, 2021

  22,624  $148.28 
Schedule of Share-based Payment Award, Awards Other Than Stock Options, Valuation Assumptions [Table Text Block]
  

Nine Months Ended September 30,

 
  

2021

  

2020

 

Risk-free interest rate

  

0.22%

   

1.41%

 

Expected life (years)

  2.87   2.88 

Expected volatility

  

51.40%

   

38.70%

 

Expected dividend yield

  

0%

   

0%

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

Nine Months Ended September 30,

 
  

2021

  

2020

 

Risk-free interest rate

  

0.06 - 0.12%

   

0.17 - 1.59%

 

Expected life (months)

  6.00   6.00 

Expected volatility

  

50.85 - 65.53%

   

42.63 - 59.99%

 

Expected dividend yield

  

0%

   

0%