Note 11 - Stock-based Compensation - Summary of Stock Option Grants and Exercises (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Proceeds from stock option exercises, net of cashless exercises	$ 3,000	$ 128,000	$ 108,000
Mercantile Plans [Member]
Aggregate intrinsic value of stock options exercised	17,000	157,000	317,000
Proceeds from stock option exercises, net of cashless exercises	3,000	126,000	141,000
Tax benefit realized from stock option exercises	$ 0	$ 0	$ 0

Amount of tax benefit from exercise of option under share-based payment arrangement.

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Amount of cash inflow from exercise of option under share-based payment arrangement.

+ References

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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-Publisher FASB
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