Note 4 - Stockholders' Equity - Stock Options, Valuation Assumptions (Details) - $ / shares	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Weighted average fair value (in dollars per share)			$ 4.22
Stock Plans [Member]
Expected life (Year)	4 years 156 days	4 years 149 days	4 years 156 days	4 years 149 days
Volatility	44.01%	41.49%	43.82%	41.53%
Risk-free interest rate	2.73%	1.65%	2.71%	1.69%
Expected dividend	0.00%	0.00%
Weighted average fair value (in dollars per share)	$ 4.21	$ 5.78	$ 4.22	$ 6.14
Employee Stock Purchase Plan [Member]
Expected life (Year)			1 year 91 days	1 year 91 days
Volatility			42.85%	40.63%
Risk-free interest rate			2.48%	1.25%
Expected dividend			0.00%	0.00%
Weighted average fair value (in dollars per share)			$ 3.62	$ 5.22

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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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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- 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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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)(i)
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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