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Note 4 - Earnings Per Share (Tables)
 3 Months Ended
Mar. 31, 2017
Notes Tables  
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]
   
Three months ended
  
   
March 31,
  
   
2017
    
2016
  
                 
Net income
   $
9,184,965
     $
6,373,489
  
                 
Basic EPS
   $
0.70
     $
0.54
  
Diluted EPS
   $
0.68
     $
0.53
  
                 
Weighted average common shares outstanding
    
13,133,382
      
11,793,620
  
Weighted average common shares issuable upon exercise of stock options and under the employee stock purchase plan
    
355,035
      
160,329
  
Weighted average common and common equivalent shares outstanding
    
13,488,417
      
11,953,949