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Note 6 - Net Income Per Share (Tables)
 9 Months Ended
Sep. 30, 2018
Notes Tables  
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]
   
Three Months Ended September 30,
    
Nine Months Ended September 30,
  
   
2018
    
2017
    
2018
    
2017
  
Numerator:
                                
Net income
   $
31,568
     $
23,642
     $
77,630
     $
53,132
  
                                 
Denominator:
                                
Weighted-average outstanding shares used to compute basic net income per share
    
42,362
      
41,458
      
42,173
      
41,276
  
Effect of dilutive securities
    
2,307
      
2,028
      
2,277
      
2,108
  
Weighted-average outstanding shares used to compute diluted net income per share
    
44,669
      
43,486
      
44,450
      
43,384
  
                                 
Net income per share:
                                
Basic
   $
0.75
     $
0.57
     $
1.84
     $
1.29
  
Diluted
   $
0.71
     $
0.54
     $
1.75
     $
1.22