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Note 1 - Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2018
Notes Tables |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | The Company uses the Black-Scholes option pricing model to estimate the fair value of stock option grants with the following assumptions for the indicated periods:
|  | 2018 |  | 2017 |  | 2016 |
Dividend yield |  |  | 0.45% to 0.48% |  |  | 0.36% to 0.47% |  |  | 0.35% to 0.51% |
Expected volatility |  |  | 29.51% to 29.59% |  |  | 29.64% to 29.95% |  |  | 29.32% to 29.37% |
Risk-free interest rate |  |  | 2.60% to 2.94% |  |  | 2.50% to 2.81% |  |  | 1.73% to 2.18% |
Expected life of option grants (in years) |  |  | 6 years |  |  | 6 years |  |  | 6 years |
Weighted-average grant date fair value |  | $ | 14.68 |  | $ | 14.75 |  | $ | 7.31 |
The Company also uses the Black-Scholes option pricing model to estimate the fair value of stock purchase grants with the following assumptions for the indicated periods:
|  | 2018 |  | 2017 |  | 2016 |
Dividend yield |  |  | 0.37% to 0.51% |  |  | 0.37% to 0.42% |  |  | 0.33% to 0.59% |
Expected volatility |  |  | 20.90% to 21.40% |  |  | 19.80% to 19.86% |  |  | 12.70% to 15.60% |
Risk-free interest rate |  |  | 1.59% to 2.22% |  |  | 0.67% to 1.18% |  |  | 0.39% to 0.57% |
Expected life of purchase grants (in months) |  |  | 3 to 6 months |  |  | 3 to 6 months |  |  | 3 to 6 months |
Weighted-average grant date fair value |  | $ | 6.63 |  | $ | 6.42 |  | $ | 3.28 |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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