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Note 15 - Stock-based Compensation (Tables)
12 Months Ended
Dec. 31, 2021
Notes Tables  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
  

Year Ended December 31,

   
  

2021

  

2020

  

2019

 

Expected dividend yield

   0%     0%     0%  

Expected volatility

  118%-139%   82%-126%   81%-92% 

Risk-free interest rate

  0.33%-1.11%   0.2%-1.69%   1.39%-2.66% 

Expected life (in years)

  3.8-4.5   3.8-6.1   5.5-6.1 
Share-based Payment Arrangement, Option, Activity [Table Text Block]
  

Number of
Shares

  

Weighted-
Average Exercise
Price

  

Weighted-Average
Remaining
Contractual Term
(Years)

 

Options outstanding at December 31, 2020

  4,361,872  $10.49   6.05 

Granted

  1,490,266   4.06     

Forfeited

  (285,391)  8.10     

Cancelled

  (610,287)  30.32     

Exercised

  (315,800)  0.67     

Options outstanding at December 31, 2021

  4,640,660  $6.64   5.66 
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable [Table Text Block]
  

Number of Shares

  

Weighted-
Average Exercise
Price

  

Weighted-Average
Remaining
Contractual Term
(Years)

  

Aggregate
Intrinsic Value
(Millions)

 

Exercisable at December 31, 2021

  1,887,155  $11.50   5.40  $0.7 

Vested or expected to vest at December 31, 2021

  4,498,953  $6.75   5.65  $1.4 
Share-based Payment Arrangement, Restricted Stock Unit, Activity [Table Text Block]
  

Number of
Restricted Stock
Units Oustanding

  

Weighted-
Average Grant
Date Fair Value

 

Unvested December 31, 2018

  382,098  $20.24 

Granted

  192,987   31.42 

Vested

  (85,153)  25.98 

Forfeited

  (46,005)  21.38 

Unvested December 31, 2019

  443,927  $23.88 

Granted

  3,112,382   0.67 

Vested

  (313,508)  19.38 

Forfeited

  (245,402)  6.54 

Unvested December 31, 2020

  2,997,399  $1.41 

Granted

  3,133,753   2.77 

Vested

  (1,891,869)  1.63 

Forfeited

  (400,253)  1.86 

Unvested December 31, 2021

  3,839,030  $2.36