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Note 13 - Stock Option Plans - Fair Value of Stock Options Granted (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Volatility	37.90%	41.60%	66.10%
Risk-free interest rate	2.70%	1.80%	1.30%
Expected life (in years) (Year)	4 years 175 days	4 years 200 days	4 years 109 days
The weighted average grant date fair value for options issued, with the exercise price equal to market value on the date of grant (in dollars per share)	$ 22.22	$ 20.08	$ 11.18

X

- 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.

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-Name Accounting Standards Codification
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-Section 50
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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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.

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