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Note 7 - Stockholders' Equity (Tables)
 3 Months Ended
Mar. 31, 2017
Notes Tables  
Share-based Compensation, Stock Options, Activity [Table Text Block]
 
 
 
 
 
 
 
 
 
 
Weighted-
 
 
 
 
 
 
 
 
 
 
 
Average
 
 
 
 
 
 
 
Weighted-
 
 
Remaining
 
 
 
 
 
 
 
Average
 
 
Contractual
 
 
 
Shares
 
 
Exercise
 
 
Term
 
 
 
(in thousands)
 
 
Price
 
 
(Years)
 
Balance at January 1, 2017
    
11,845
     $
14.44
      
7.60
  
Granted
    
416
     $
13.65
      
 
  
Exercised
    
(197
)
   $
9.38
      
 
  
Expired and forfeited
    
(215
)
   $
19.73
      
 
  
Balance at March 31, 2017
    
11,849
     $
14.40
      
7.47
  
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]
 
 
Three Months Ended
March 31,
 
 
 
2017
 
 
2016
 
                 
Research and development
   $
3,208
     $
2,073
  
General and administrative
    
2,374
      
1,667
  
Sales and marketing
    
2,439
      
1,615
  
Stock-based compensation expense included in operating expenses
   $
8,021
     $
5,355
  
                 
Impact on basic and diluted net loss per share
   $
0.16
     $
0.15
  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
 
 
Three Months Ended
March 31,
 
 
 
20
17
 
 
20
16
 
Risk-free interest rate
    
2.1
%    
1.6
 %
Dividend yield
    
0.0
 %    
0.0
 %
Volatility
    
87.3
 %    
90.6
 %
Expected life (years)
    
6
      
6