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Note 7 - Stock-based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2019
Notes Tables |
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block] |
|  | Year Ended December 31,
|  | 2019 |  | 2018 |  | 2017
Research and development |  | $ | 13,977 |  |  | $ | 6,867 |  |  | $ | 9,602 |
General and administrative |  |  | 3,742 |  |  |  | 2,529 |  |  |  | 3,019 |
Total stock-based compensation expense |  | $ | 17,719 |  |  | $ | 9,396 |  |  | $ | 12,621 |
Share-based Payment Arrangement, Activity [Table Text Block] |
|  | Awards Available |  | Options Outstanding |  | Weighted Average Exercise Price
Balance at December 31, 2016 |  |  | 2,273 |  |  |  | 12,095 |  |  | $ | 6.55 |
Plan amendment |  |  | 1,000 |  |  |  | - |  |  |  | - |
Restricted stock awards granted |  |  | (22 | ) |  |  | - |  |  |  | - |
Restricted stock awards cancelled |  |  | 12 |  |  |  | - |  |  |  | - |
Stock option awards granted |  |  | (3,915 | ) |  |  | 3,915 |  |  |  | 5.33 |
Stock option awards exercised |  |  | - |  |  |  | (438 | ) |  |  | 3.50 |
Stock option awards cancelled |  |  | 1,120 |  |  |  | (1,120 | ) |  |  | 9.72 |
Balance at December 31, 2017 |  |  | 468 |  |  |  | 14,452 |  |  |  | 6.06 |
Plan amendment |  |  | 4,400 |  |  |  | - |  |  |  | - |
Restricted stock awards granted |  |  | (13 | ) |  |  | - |  |  |  | - |
Stock option awards granted |  |  | (4,272 | ) |  |  | 4,272 |  |  |  | 7.15 |
Stock option awards exercised |  |  | - |  |  |  | (1,011 | ) |  |  | 2.92 |
Stock option awards cancelled |  |  | 222 |  |  |  | (222 | ) |  |  | 7.44 |
Balance at December 31, 2018 |  |  | 805 |  |  |  | 17,491 |  |  |  | 6.49 |
Plan amendment |  |  | 4,000 |  |  |  | - |  |  |  | - |
Restricted stock awards granted |  |  | (27 | ) |  |  | - |  |  |  | - |
Stock option awards granted |  |  | (4,511 | ) |  |  | 4,511 |  |  |  | 3.91 |
Stock option awards exercised |  |  | - |  |  |  | (251 | ) |  |  | 3.75 |
Stock option awards cancelled |  |  | 701 |  |  |  | (701 | ) |  |  | 6.82 |
Balance at December 31, 2019 |  |  | 968 |  |  |  | 21,050 |  |  | $ | 5.96 |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] |
|  | 2019 |  | 2018 |  | 2017
Expected Life |  |  | 5.5 |  |  |  | 5.5 |  |  |  | 5.5 |
Expected Volatility |  |  | 81 | % |  |  | 82 | % |  |  | 82 | %
Expected Dividend Yield |  |  | 0.0 | % |  |  | 0.0 | % |  |  | 0.0 | %
Risk-Free Interest Rate |  |  | 1.8 | % |  |  | 2.7 | % |  |  | 2.0 | %
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding and Exercisable [Table Text Block] |
|  | Outstanding |  | Exercisable
Range |  | Number |  | Weighted Average Remaining Life |  | Weighted Average Exercise Price |  | Number |  | Weighted Average Exercise Price
$ | 0 | to | 3 |  | 1,929 |  |  | 5.5 |  |  | $ | 2.02 |  |  |  | 1,159 |  |  | $ | 1.64 |
| 3 | to | 6 |  | 11,724 |  |  | 7.2 |  |  |  | 4.17 |  |  |  | 5,085 |  |  |  | 4.55 |
| 6 | to | 9 |  | 4,941 |  |  | 7.7 |  |  |  | 7.14 |  |  |  | 1,830 |  |  |  | 6.98 |
| 9 | to | 12 |  | 3,086 |  |  | 5.3 |  |  |  | 10.78 |  |  |  | 2,013 |  |  |  | 10.91 |
| 12 | to | 15 |  | 604 |  |  | 5.1 |  |  |  | 12.29 |  |  |  | 509 |  |  |  | 12.31 |
| 15 | to | 18 |  | 95 |  |  | 5.5 |  |  |  | 15.39 |  |  |  | 95 |  |  |  | 15.39 |
$ | 0 | to | 18 |  | 22,379 |  |  | 6.9 |  |  | $ | 5.82 |  |  |  | 10,691 |  |  | $ | 6.31 |
Share-based Payment Arrangement, Option, Activity [Table Text Block] |
|  | Non-Vested Stock Option Awards |  | Weighted Average Grant-Date Fair Value
Balance December 31, 2018 |  |  | 9,295 |  |  | $ | 4.78 |
Stock option awards granted |  |  | 5,891 |  |  |  | 2.58 |
Stock option awards vested |  |  | (2,794 | ) |  |  | 4.48 |
Stock option awards forfeited |  |  | (704 | ) |  |  | 4.38 |
Balance December 31, 2019 |  |  | 11,688 |  |  | $ | 3.77 |

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- Definition Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5047-113901 + Details
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