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Note 6 - Stock-based Compensation (Tables)
6 Months Ended
Jun. 30, 2021
Notes Tables  
Share-based Payment Arrangement, Restricted Stock Unit, Activity [Table Text Block]
  

Time-Based RSUs

  

Performance-Based RSUs

 
      

Weighted-Average

      

Weighted-Average

 
      

Grant Date

      

Grant Date

 
  

Shares

  

Fair Value

  

Shares

  

Fair Value

 

Balance at December 31, 2020

  292,670  $33.37   302,802  $32.55 

Granted

  60,666   41.11       

Vested

  (93,856)  41.87   (76,292)  42.56 

Forfeited

  (23,324)  29.92   (7,768)  40.60 

Balance at June 30, 2021

  236,156  $32.52   218,742  $28.77