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Note 8 - Stock-based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2020
Notes Tables |
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block] |
|  | Year Ended December 31, |
|  | 20 20 |  |  | 201 9 |  |  | 201 8 |
Research and development |  | $ | 10,222 |  |  | $ | 13,977 |  |  | $ | 6,867 |
Selling, general and administrative |  |  | 4,572 |  |  |  | 3,742 |  |  |  | 2,529 |
Total stock-based compensation expense |  | $ | 14,794 |  |  | $ | 17,719 |  |  | $ | 9,396 |
Share-based Payment Arrangement, Activity [Table Text Block] |
|  |  |  |  |  |  |  |  |  | Weighted |
|  |  |  |  |  |  |  |  |  | Average |
|  | Awards |  |  | Options |  |  | Exercise |
|  | Available |  |  | Outstanding |  |  | Price |
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 |
Plan amendment |  |  | 8,000 |  |  |  | − |  |  |  | − |
Restricted stock awards granted |  |  | (31 | ) |  |  | − |  |  |  | − |
Stock option awards granted |  |  | (7,469 | ) |  |  | 7,469 |  |  |  | 8.06 |
Stock option awards exercised |  |  | − |  |  |  | (510 | ) |  |  | 3.56 |
Stock option awards cancelled |  |  | 3,124 |  |  |  | (3,124 | ) |  |  | 6.93 |
Balance at December 31, 2020 |  |  | 4,592 |  |  |  | 24,885 |  |  | $ | 6.52 |
|  |  |  |  |  |  |  |  |  | Weighted |
|  |  |  |  |  |  |  |  |  | Average |
|  | Awards |  |  | Options |  |  | Exercise |
|  | Available |  |  | Outstanding |  |  | Price |
Balance at December 31, 2019 |  |  | 171 |  |  |  | 1,329 |  |  | $ | 3.60 |
Plan amendment |  |  | 2,900 |  |  |  | − |  |  |  | − |
Stock option awards granted |  |  | (3,002 | ) |  |  | 3,002 |  |  |  | 4.02 |
Stock option awards cancelled |  |  | 160 |  |  |  | (160 | ) |  |  | 4.15 |
Balance at December 31, 2020 |  |  | 229 |  |  |  | 4,171 |  |  | $ | 3.88 |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] |
|  | 20 20 |  |  | 201 9 |  |  | 201 8 |
Expected Life |  |  | 5.5 |  |  |  | 5.5 |  |  |  | 5.5 |
Expected Volatility |  |  | 84 | % |  |  | 81 | % |  |  | 82 | %
Expected Dividend Yield |  |  | 0.0 | % |  |  | 0.0 | % |  |  | 0.0 | %
Risk-Free Interest Rate |  |  | 0.4 | % |  |  | 1.8 | % |  |  | 2.7 | %
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding and Exercisable [Table Text Block] |
|  |  |  |  |  |  |  |  | Outstanding |  |  | Exercisable |
|  |  |  |  |  |  |  |  |  |  |  | Weighted |  |  | Weighted |  |  |  |  |  |  | Weighted |
|  |  |  |  |  |  |  |  |  |  |  | Average |  |  | Average |  |  |  |  |  |  | Average |
|  |  |  |  |  |  |  |  |  |  |  | Remaining |  |  | Exercise |  |  |  |  |  |  | Exercise |
Range |  |  | Number |  |  | Life |  |  | Price |  |  | Number |  |  | Price |
$ | 0 |  | to |  | 3 |  |  |  | 2,514 |  |  |  | 6.1 |  |  | $ | 2.19 |  |  |  | 1,414 |  |  | $ | 1.25 |
| 3 |  | to |  | 6 |  |  |  | 12,689 |  |  |  | 7.0 |  |  |  | 4.22 |  |  |  | 6,854 |  |  |  | 2.48 |
| 6 |  | to |  | 9 |  |  |  | 10,852 |  |  |  | 9.2 |  |  |  | 7.88 |  |  |  | 2,005 |  |  |  | 4.89 |
| 9 |  | to |  | 12 |  |  |  | 2,347 |  |  |  | 4.5 |  |  |  | 10.85 |  |  |  | 2,129 |  |  |  | 4.51 |
| 12 |  | to |  | 15 |  |  |  | 559 |  |  |  | 4.1 |  |  |  | 12.30 |  |  |  | 539 |  |  |  | 6.43 |
| 15 |  | to |  | 18 |  |  |  | 95 |  |  |  | 4.5 |  |  |  | 15.39 |  |  |  | 95 |  |  |  | 10.57 |
$ | 0 |  | to |  | 18 |  |  |  | 29,056 |  |  |  | 7.5 |  |  | $ | 6.14 |  |  |  | 13,036 |  |  | $ | 3.27 |
Share-based Payment Arrangement, Option, Activity [Table Text Block] |
|  | Non-Vested Stock Option Awards |  |  | Weighted Average Grant-Date Fair Value |
Balance December 31, 2019 |  |  | 11,688 |  |  | $ | 3.77 |
Stock option awards granted |  |  | 10,471 |  |  |  | 4.69 |
Stock option awards vested |  |  | (4,543 | ) |  |  | 4.12 |
Stock option awards forfeited |  |  | (1,596 | ) |  |  | 3.96 |
Balance December 31, 2020 |  |  | 16,020 |  |  | $ | 4.25 |

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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=120381028&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=120381028&loc=d3e5047-113901 + Details
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- Definition Tabular disclosure of activity for award under share-based payment arrangement. Includes, but is not limited to, outstanding award at beginning and end of year, granted, exercised, forfeited, and weighted-average grant date fair value. + 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 (e) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + 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 (f)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of number, weighted-average exercise price or conversion ratio, aggregate intrinsic value, and weighted-average remaining contractual term for outstanding and exercisable options that are fully vested and expected to vest. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur. + 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 (e) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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