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Note 7 - Stock-based Compensation (Tables)
 9 Months Ended
Sep. 30, 2017
Notes Tables  
Share-based Compensation, Stock Options, Activity [Table Text Block]
    Options   Weighted-
Average
Exercise
Price		   Weighted-
Average
Remaining
Contractual
Term (years)		   Aggregate
Intrinsic
Value
                 
Outstanding at December 31, 2016    
2,166,254
     $
8.55
      
7.1
     $
124
  
Granted    
719,337
      
 
      
 
      
 
  
Exercised    
(74,001
 )    
 
      
 
      
 
  
Forfeited    
(30,272
 )    
 
      
 
      
 
  
Expired    
(97,406
 )    
 
      
 
      
 
  
Outstanding at September 30, 2017    
2,683,912
     $
7.18
      
7.4
     $
140
  
Exercisable at September 30, 2017    
1,384,925
     $
8.00
      
6.2
     $
98
  
Vested or expected to vest at September 30, 2017 (1)    
2,683,912
     $
7.18
      
7.4
     $
140
  
Common Stock Reserved for Future Issuance [Table Text Block]
    September 30,
2017		   December 31,
2016
         
Stock-based compensation awards    
3,587,144
      
2,982,470
  
Employee Stock Purchase Plan    
247,774
      
292,821
  
Total    
3,834,918
      
3,275,291