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Note 15 - Stock-based Compensation - Fair Value of Each Option Using Black-scholes Valuation Model Assumptions (Details) |  |  |  |  | 12 Months Ended | 36 Months Ended
--- | --- | --- | --- | --- | --- | ---
Apr. 02, 2017 | Nov. 03, 2015 | Nov. 05, 2014 | Feb. 11, 2014 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Apr. 02, 2017
Risk-free interest rates |  |  |  |  | 1.90% | 1.30% | 1.40% |
Expected life (in years) (Year) |  |  |  |  | 3 years 36 days | 4 years 182 days | 4 years |
Dividend yield |  |  |  |  | 0.62% | 1.10% | 0.70% |
Expected volatility |  |  |  |  | 27.20% | 30.70% | 29.20% |
Chief Executive Officer [Member] | Options to Purchase 300,000 Shares of Common Stock [Member] |  |  |  |  |  |  |  |
Risk-free interest rates |  |  |  |  |  |  |  | 2.36%
Expected life (in years) (Year) |  |  |  |  |  |  |  | 7 years 91 days
Dividend yield |  |  |  |  |  |  |  | 0.90%
Expected volatility |  |  |  |  |  |  |  | 42.80%
Chief Executive Officer [Member] | Options to Purchase 100,000 Shares of Common Stock [Member] |  |  |  |  |  |  |  |
Risk-free interest rates | 1.64% |  |  |  |  |  |  |
Expected life (in years) (Year) | 4 years 273 days |  |  |  |  |  |  |
Dividend yield | 0.90% |  |  |  |  |  |  |
Expected volatility | 33.10% |  |  |  |  |  |  |
2012 Incentive Plan [Member] | Chief Financial Officer [Member] |  |  |  |  |  |  |  |
Risk-free interest rates |  | 1.35% | 1.30% | 0.81% |  |  |  |
Expected life (in years) (Year) |  | 4 years | 4 years | 3 years 136 days |  |  |  |
Dividend yield |  | 0.70% | 0.70% | 0.80% |  |  |  |
Expected volatility |  | 29.20% | 32.40% | 33.50% |  |  |  |
Forfeiture rate |  | 0.00% | 0.00% | 0.00% |  |  |  |

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- Definition The forfeiture rate assumption that is used in valuing an option on its own shares. + References No definition available. + Details
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- Definition The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iv) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.D.2) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 + Details
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