Note 9 - Share-based Compensation (Details Textual) - USD ($)	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Share-based Payment Arrangement, Amount Capitalized	$ 0	$ 0
Share-based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount	$ 5,200,000
Share-based Payment Arrangement, Option [Member]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	2 years 73 days
Restricted Stock Units (RSUs) [Member]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	292 days
Share-based Payment Arrangement, Nonvested Award, Excluding Option, Cost Not yet Recognized, Amount	$ 235,000

Amount of cost capitalized for award under share-based payment arrangement.

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 (h)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Section 50
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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Amount of cost to be recognized for option under share-based payment arrangement.

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-Section 50
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