Note 11 - Equity Plans - Restricted Stock Unit Activity (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares	3 Months Ended	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Outstanding, number of shares (in shares)	636,086	527,546	527,546
Outstanding, weighted average estimated fair value (in dollars per share)	$ 19.25	$ 20.65	$ 20.65
Granted, number of shares (in shares)	70,588	156,562
Granted, weighted average estimated fair value (in dollars per share)	$ 11.92	$ 17.40
Converted into shares, number of shares (in shares)	(5,329)
Converted into shares, weighted average estimated fair value (in dollars per share)	$ 15.94
Forfeited, number of shares (in shares)	(494)
Forfeited, weighted average estimated fair value (in dollars per share)	$ 27.31
Outstanding, number of shares (in shares)	700,851	684,108	636,086
Outstanding, weighted average estimated fair value (in dollars per share)	$ 18.53	$ 19.91	$ 19.25
Vested, number of shares (in shares)	445,781	515,778
Vested, weighted average estimated fair value (in dollars per share)	$ 20.22	$ 20.41

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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 (c)(1)(iv)(3)
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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Number of shares issued under share-based payment arrangement.

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The weighted average fair value at grant date for converted equity-based awards issued during the period on other than stock (or unit) option plans.

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