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Note 6 - Stock-based Compensation (Tables)
 3 Months Ended
Mar. 31, 2019
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, 2018
    
284,493
     $
35.79
      
265,659
     $
32.90
  
Granted
    
18,818
      
38.36
      
—
      
—
  
Vested
    
(21,040
 )    
36.03
      
(57,409
 )    
38.36
  
Forfeited
    
(8,923
 )    
37.12
      
—
      
—
  
Balance at March 31, 2019
 
 
273,348
    
$
35.91
    
 
208,250
    
$
31.39