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Note 8 - Share-based Compensation (Details Textual) | 9 Months Ended
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Sep. 30, 2018 USD ($) $ / shares shares | Sep. 30, 2017 $ / shares shares
Number Of Active Stock Incentive Plans | 1 |
Number Of Inactive Stock Option Plans | 1 |
Restricted Stock Units and Performance Share Award Target Level, Percentage | 100.00% |
Director [Member] |  |
Share-based Goods and Nonemployee Services Transaction, Quantity of Securities Issued | 11,172 | 14,944
Share-based Compensation Arrangement by Stock-based Payment Award, Grant Date Fair Value | $ / shares | $ 21.48 | $ 14.72
Minimum [Member] |  |
Performance Awards Issued Multiplier | 0.00% |
Maximum [Member] |  |
Performance Awards Issued Multiplier | 200.00% |
Restricted Stock Units (RSUs) [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | 3 years |
Performance Shares [Member] |  |
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Number At Target Level Of Performance | 40,000 |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options | $ | $ 0 |

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- Definition Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation. + 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 (i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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