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Note 8 - Stockholders' Equity (Tables)
3 Months Ended
Mar. 31, 2018
Notes Tables  
Share-based Compensation, Stock Options, Activity [Table Text Block]
   
 
 
 
 
 
 
 
 
Weighted-
 
   
 
 
 
 
 
 
 
 
a
verage
 
   
 
 
 
 
Weighted-
   
Remaining
 
   
 
 
 
 
a
verage
   
Contractual
 
   
Shares
   
Exercise
   
Term
 
   
(in thousands)
   
Price
   
(Years)
 
Balance at December 31, 2017
   
13,463
    $
15.03
     
8.01
 
Granted
   
122
    $
23.06
     
 
 
Exercised
   
(435
)
  $
9.66
     
 
 
Expired and forfeited
   
(244
)
  $
19.02
     
 
 
Balance at March 31, 2018
   
12,906
    $
15.22
     
7.86
 
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]
   
Three Months Ended
March 31,
 
   
2018
   
2017
 
                 
Research and development
  $
3,036
    $
3,208
 
General and administrative
   
2,276
     
2,374
 
Sales and marketing
   
2,389
     
2,439
 
Total stock-based compensation expense
  $
7,701
    $
8,021
 
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
   
March 31
,
 
   
20
18
   
20
17
 
Risk-free interest rate
   
2.6
%    
2.1
%
Dividend yield
   
0.0
%    
0.0
%
Volatility
   
71.4
%    
87.3
%
Expected life (years)
   
6
     
6