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Note 13 - Stock-based Compensation - Assumptions Used in the Black-Scholes Option Pricing Model (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Expected dividend yield	0.00%
Employee Stock Option [Member]
Expected life (Year)	6 years 91 days	6 years 182 days	6 years 182 days
Expected dividend yield	0.00%	0.00%	0.00%
Weighted average grant date fair value (in dollars per share)	$ 50.08	$ 32.26	$ 26.61
Employee Stock Option [Member] | Minimum [Member]
Risk-free interest rate	2.52%	2.24%	1.53%
Expected volatility	41.68%	42.68%	44.38%
Employee Stock Option [Member] | Maximum [Member]
Risk-free interest rate	3.07%	2.36%	2.68%
Expected volatility	42.22%	44.68%	45.93%

X

- 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

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-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=109197908&loc=d3e5070-113901

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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.

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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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