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Note 12 - Stock Option and Equity Incentive Plans - Black-Scholes Option Pricing Model (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Expected volatility	32.80%	28.90%	27.70%
Expected dividends	0.00%	0.00%	0.00%
Risk-free interest rate	0.30%	2.30%	2.70%
Exercise price (in dollars per share)	$ 43.95	$ 38.61	$ 31.20
Expected term (years) (Year)	6 years 1 month 6 days	6 years	6 years
Weighted-average grant date fair value (in dollars per share)	$ 14.10	$ 12.70	$ 10.15

X

- Definition

Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

+ References

No definition available.

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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

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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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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- Definition

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

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- Definition

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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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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