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Note 6 - Share-based Payments - Amounts of Compensation Expense Recognized Under Stock-based Payment Programs (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Stock compensation expense | $ 1,237 | $ 2,148 | $ 4,007 | $ 3,944
Stock Performance Awards [Member] | Executive Officer [Member] |  |  |  |
Stock compensation expense | 712 | 1,418 | 2,412 | 2,531
Restricted Stock Units (RSUs) [Member] | Executive Officers and Key Employees [Member] |  |  |  |
Stock compensation expense | 106 | 383 | 785 | 810
Restricted Stock Units (RSUs) [Member] | Nonemployee Directors [Member] |  |  |  |
Stock compensation expense | 236 | 204 | 440 | 369
Restricted Stock Units (RSUs) [Member] | Non-executive Employees [Member] |  |  |  |
Stock compensation expense | 141 | 143 | 275 | 234
Employee Stock Purchase Plan [Member] |  |  |  |
Stock compensation expense | $ 42 | $ 0 | $ 95 | $ 0

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