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Note 7 - Stockholders' Equity (Tables)
 9 Months Ended
Sep. 30, 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
    
1,218
     $
14.45
      
 
  
Exercised
    
(952
)
   $
8.23
      
 
  
Expired and forfeited
    
(913
)
   $
19.53
      
 
  
Balance at
September 30, 2017
    
11,198
     $
14.56
      
7.32
  
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]
   
Three Months Ended
September
30
,
    
Nine
Months Ended
September
30,
  
   
2017
    
2016
    
2017
    
2016
  
                                 
Research and development
   $
2,595
     $
3,946
     $
8,758
     $
8,347
  
General and administrative
    
2,419
      
1,763
      
7,345
      
5,229
  
Sales and marketing
    
2,462
      
1,802
      
7,544
      
5,121
  
Stock
-based compensation expense included in operating expenses
   $
7,476
     $
7,511
     $
23,647
     $
18,697
  
                                 
Impact on basic and diluted net loss per share
   $
0.14
     $
0.19
     $
0.45
     $
0.50
  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
   
September
30
,
  
   
20
17
    
20
16
  
Risk-free interest rate
    
2.0
 %    
1.5
 %
Dividend yield
    
0.0
 %    
0.0
 %
Volatility
    
80.2
 %    
90.6
 %
Expected life (years)
    
6
      
6