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Note 13 - Net Loss Per Share of Common Stock (Tables)
12 Months Ended
Dec. 31, 2019
Notes Tables  
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]
   
Year Ended December 31,
 
   
201
9
   
201
8
   
201
7
 
ESPP, RSUs and stock options to purchase common stock
   
13,798,797
     
11,797,960
     
8,767,783
 
Common stock warrants
   
176,679
     
     
176,730