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Note 7 - Stockholders' Equity (Tables)
 6 Months Ended
Jun. 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
    
1,022
     $
14.21
      
 
  
Exercised
    
(315
)
   $
8.45
      
 
  
Expired and forfeited
    
(586
)
   $
18.89
      
 
  
Balance at June 30, 2017
    
11,966
     $
14.36
      
7.18
  
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]
 
 
Three Months Ended
June 30
,
 
 
Six Months Ended
June 30,
 
 
 
2017
 
 
2016
 
 
2017
 
 
2016
 
                                 
Research and development
   $
2,955
     $
2,329
     $
6,163
     $
4,402
  
General and administrative
    
2,552
      
1,798
      
4,926
      
3,465
  
Sales and marketing
    
2,643
      
1,704
      
5,082
      
3,319
  
Stock-based compensation expense included in operating expenses
   $
8,150
     $
5,831
     $
16,171
     $
11,186
  
                                 
Impact on basic and diluted net loss per share
   $
0.15
     $
0.16
     $
0.31
     $
0.31
  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
 
 
June 30,
 
 
 
20
17
 
 
20
16
 
Risk-free interest rate
    
2.0
 %    
1.5
 %
Dividend yield
    
0.0
 %    
0.0
 %
Volatility
    
81.4
 %    
90.6
 %
Expected life (years)
    
6
      
6
  
 
 
June 30,
 
 
 
20
17
 
 
20
16
 
Risk-free interest rate
    
1.0
 %    
0.4
 %
Dividend yield
    
0.0
 %    
0.0
 %
Volatility
    
56.0
 %    
85.1
 %
Expected life (months)
    
6
      
6