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Note 7 - Stockholders' Equity (Details Textual) - USD ($) $ / shares in Units, $ in Thousands |  |  |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 01, 2020 | Nov. 21, 2019 | Nov. 18, 2019 | Aug. 06, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Apr. 24, 2020 | Nov. 08, 2017
Maximum Aggregate Offering Price |  |  |  |  |  |  |  | $ 500,000 | $ 200,000
Stock Issued During Period, Shares, New Issues (in shares) | 22,044 |  | 43,621,000 | 10,455,000 |  |  |  |  |
Shares Issued, Price Per Share (in dollars per share) | $ 4.50 | $ 1.69 | $ 1.45 | $ 5.50 |  |  |  |  |
Proceeds from Issuance of Common Stock |  |  | $ 58,500 | $ 53,400 | $ 93,279 | $ 58,500 | $ 53,400 |  |
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares) |  | 11,765,000 |  |  |  |  |  |  |
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) |  | $ 0.01 |  |  |  |  |  |  |
Proceeds from Issuance of Warrants |  | $ 19,882 |  |  | $ 14,817 | $ 19,882 |  |  |
Proceeds from Issuance or Sale of Equity, Total | $ 108,096,000 |  |  |  |  |  |  |  |
Warrants Issued in Public Offering [Member] |  |  |  |  |  |  |  |  |
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares) | 3,511 |  |  |  |  |  |  |  |
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ 0.01 |  |  |  |  |  |  |  |
Class of Warrant or Right, Warrants Issued, Price Per Warrant (in dollars per share) | $ 4.49 |  |  |  |  |  |  |  |

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