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Note 9 - Fair Value of Financial Instruments (Details Textual) - USD ($)
Jun. 30, 2020
Dec. 31, 2019
Warrants and Rights Outstanding $ 7,752,534 $ 6,116,882
Fair Value, Inputs, Level 1 [Member]    
Cash and Cash Equivalents, Fair Value Disclosure 41,900,000 56,700,000
Restricted Cash and Cash Equivalents, Total   5,600,000
Fair Value, Inputs, Level 2 [Member]    
Cash and Cash Equivalents, Fair Value Disclosure 0  
Restricted Cash and Cash Equivalents, Total   $ 90,000,000.0
The 2016 Warrant [Member]    
Warrants and Rights Outstanding $ 7,800,000