Note 14 - Stock-based Compensation - Fair Value Assumptions of Stock Options and ESOP (Details)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Minimum [Member] \| Employee Stock Purchase Plan [Member]
Risk-free interest rate	1.48%	0.51%	0.22%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	91 days	91 days	91 days
Volatility	29.00%	40.00%	34.00%
Maximum [Member] \| Employee Stock Purchase Plan [Member]
Risk-free interest rate	2.32%	1.18%	0.35%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	94 days	94 days	94 days
Volatility	59.00%	79.00%	48.00%
Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	5 years	5 years	5 years
Volatility			34.00%
Share-based Payment Arrangement, Option [Member] \| Minimum [Member]
Risk-free interest rate	2.51%	1.84%	1.14%
Volatility	39.00%	37.00%
Share-based Payment Arrangement, Option [Member] \| Maximum [Member]
Risk-free interest rate	2.85%	1.93%	1.33%
Volatility	42.00%	38.00%

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.

+ References

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- Definition

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.

+ References

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)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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=115993241&loc=d3e301413-122809

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