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Note 8 - Stock-based Compensation - Stock Plan Activities (Details) - $ / shares shares in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Incentive Plan [Member] |  |  |
Awards available, beginning balance (in shares) | 968 | 805 | 468
Options outstanding, beginning balance (in shares) | 21,050 | 17,491 | 14,452
Weighted average exercise price, beginning balance (in dollars per share) |  |  | $ 6.06
Awards available, plan amendment (in shares) | 8,000 | 4,000 | 4,400
Awards available, Restricted stock awards granted (in shares) | (31) | (27) | (13)
Awards available, stock option awards granted (in shares) | (7,469) | (4,511) | (4,272)
Options outstanding, stock option awards granted (in shares) | 7,469 | 4,511 | 4,272
Weighted average exercise price, stock option awards granted (in dollars per share) | $ 8.06 | $ 3.91 | $ 7.15
Options outstanding, stock option awards exercised (in shares) | (510) | (251) | (1,011)
Weighted average exercise price, stock option awards exercised (in dollars per share) | $ 3.56 | $ 3.75 | $ 2.92
Awards available, stock option awards cancelled (in shares) | 3,124 | 701 | 222
Options outstanding, stock option awards cancelled (in shares) | (3,124) | (701) | (222)
Weighted average exercise price, stock option awards cancelled (in dollars per share) | $ 6.93 | $ 6.82 | $ 7.44
Awards available, ending balance (in shares) | 4,592 | 968 | 805
Options outstanding, ending balance (in shares) | 24,885 | 21,050 | 17,491
Weighted average exercise price, ending balance (in dollars per share) | $ 6.52 | $ 5.96 | $ 6.49
Inducement Plan [Member] |  |  |
Awards available, beginning balance (in shares) | 171 |  |
Options outstanding, beginning balance (in shares) | 1,329 |  |
Weighted average exercise price, beginning balance (in dollars per share) | $ 3.60 |  |
Awards available, plan amendment (in shares) | 2,900 |  |
Awards available, stock option awards granted (in shares) | (3,002) |  |
Options outstanding, stock option awards granted (in shares) | 3,002 |  |
Weighted average exercise price, stock option awards granted (in dollars per share) | $ 4.02 |  |
Awards available, stock option awards cancelled (in shares) | 160 |  |
Options outstanding, stock option awards cancelled (in shares) | (160) |  |
Weighted average exercise price, stock option awards cancelled (in dollars per share) | $ 4.15 |  |
Awards available, ending balance (in shares) | 229 | 171 |
Options outstanding, ending balance (in shares) | 4,171 | 1,329 |
Weighted average exercise price, ending balance (in dollars per share) | $ 3.88 | $ 3.60 |

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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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 (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + 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 -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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