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Note 8 - Fair Value Measurements (Tables)
12 Months Ended
Dec. 31, 2019
Notes Tables  
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]
Description
 
Balance at
December 31,
2017
   
Established
in 2018
   
Change in
Fair Value
   
Balance at
December 31,
2018
 
                                 
Warrant liability
  $
15,242,915
    $
-
    $
(12,705,598
)
  $
2,537,317
 
Description
 
Balance at
December 31,
2018
   
Established
in 2019
   
Change in
Fair Value
   
Balance at
December 31,
2019
 
                                 
Warrant liability
  $
2,537,317
    $
-
    $
5,710,362
    $
8,247,679
 
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]
   
Level 1
   
Level 2
   
Level 3
   
Total
 
                                 
Warrant liability
  $
-
    $
-
    $
2,537,317
    $
2,537,317
 
Balance at December 31, 2018
  $
-
    $
-
    $
2,537,317
    $
2,537,317
 
   
Level 1
   
Level 2
   
Level 3
   
Total
 
                                 
Warrant liability
  $
-
    $
-
    $
8,247,679
    $
8,247,679
 
Balance at December 31, 2019
  $
-
    $
-
    $
8,247,679
    $
8,247,679