Note 4 - Stockholders' Equity - Stock Options, Valuation Assumptions (Details) - $ / shares	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Weighted average fair value (in dollars per share)	$ 7.84
Employee Stock Purchase Plan [Member]
Expected life (Year)	1 year 91 days	1 year 91 days
Volatility	33.32%	44.15%
Risk-free interest rate	0.93%	0.50%
Expected dividend
Weighted average fair value (in dollars per share)	$ 6.78	$ 3.56
Stock Plans [Member]
Expected life (Year)	4 years 149 days	4 years 153 days
Volatility	40.57%	44.74%
Risk-free interest rate	1.87%	1.21%
Expected dividend	0.00%	0.00%
Weighted average fair value (in dollars per share)	$ 7.85	$ 4.08

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.

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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.

+ References

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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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- 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.

+ 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)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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