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Note 9 - Share-based Compensation (Details Textual) $ / shares in Units, $ in Millions | 6 Months Ended
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Jun. 30, 2020 USD ($) $ / shares shares | Jun. 30, 2019 $ / shares shares
Number Of Active Stock Incentive Plans | 1 |
Director [Member] |  |
Share-based Goods and Nonemployee Services Transaction, Quantity of Securities Issued (in shares) | 12,815 | 9,772
Share-based Compensation Arrangement by Stock-based Payment Award, Grant Date Fair Value (in dollars per share) | $ / shares | $ 25.37 | $ 24.56
Share-based Payment Arrangement, Option [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Month) | 60 months |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year) | 10 years |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (in shares) | 24,000 |
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price (in dollars per share) | $ / shares | $ 24.15 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance (in shares) | 0 |
Restricted Stock Units (RSUs) [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Month) | 3 years |
Performance Shares [Member] |  |
Restricted Stock Units and Performance Share Award Target Level, Percentage | 100.00% |
Performance Awards Issued Multiplier | 136.00% |
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Number At Target Level Of Performance (in shares) | 91,000 |
Performance Shares [Member] | Minimum [Member] |  |
Performance Awards Issued Multiplier | 0.00% |
Performance Shares [Member] | Maximum [Member] |  |
Performance Awards Issued Multiplier | 200.00% |
Restricted Stock Units and Performance Stock Awards [Member] |  |
Share-based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount | $ | $ 3.5 |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) | 1 year 9 months 18 days |

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