Note 6 - Stock-based Compensation (Details Textual) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
Jan. 01, 2019	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		0	0
Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value		$ 1.77
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value		$ 0.2	$ 0.3
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total		$ 0.6
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition		1 year 186 days
Equity Incentive Plan 2015 [Member]
Percentage of Total Shares Eligible for Plan Reserve, On an Annual Basis		4.00%
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	135,049
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant		77,340

The percentage of total shares within the plan that may be added to the plan reserve, occurring on an annual basis.

No definition available.

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- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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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.

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- Definition

Number of additional shares authorized for issuance under share-based payment arrangement.

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- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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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

Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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- Definition

Number of share options (or share units) exercised during the current period.

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