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Note 1 - Nature of Business and Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2017
Notes Tables |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] |
|  | 2017 |  |  | 2016 |  |  | 201 5 |
Dividend yield |  |  | 0.36% | to | 0.47% |  |  |  | 0.35% | to | 0.51% |  |  |  | 0.37% | to | 0.46% |
Expected volatilit y |  |  | 29.64% | to | 29.95% |  |  |  | 29.32% | to | 29.37% |  |  |  | 28.92% | to | 29.32% |
Risk-free interest rate |  |  | 2.50% | to | 2.81% |  |  |  | 1.73% | to | 2.18% |  |  |  | 1.89% | to | 2.37% |
Expected life of option grants (in years) |  |  |  | 6 |  |  |  |  |  | 6 |  |  |  |  |  | 6 |  |
Weighted-average grant date fair value |  |  |  | $14.75 |  |  |  |  |  | $7.31 |  |  |  |  |  | $5.11 |  |
|  | 2017 |  |  | 2016 |  |  | 20 15 |
Dividend yield |  |  | 0.37% | to | 0.42% |  |  |  | 0.33% | to | 0.59% |  |  |  | 0.37% | to | 0.45% |
Expect ed volatility |  |  | 19.80% | to | 19.86% |  |  |  | 12.70% | to | 15.60% |  |  |  | 8.81% | to | 13.10% |
Risk-free interest rate |  |  | 0.67% | to | 1.18% |  |  |  | 0.39% | to | 0.57% |  |  |  | 0.09% | to | .016% |
Expected life of purchase grants (in months) |  |  | 3 | to | 6 |  |  |  | 3 | to | 6 |  |  |  | 3 | to | 6 |
Weighted-average grant date fair value |  |  |  | $6.42 |  |  |  |  |  | $3.28 |  |  |  |  |  | $2.39 |  |

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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + 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) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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