Note 7 - Stock-based Compensation (Details Textual) - USD ($)	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Payment Arrangement, Expense	$ 62,000	$ 50,000	$ 195,000	$ 150,000
Share-based Payment Arrangement, Expense, Tax Benefit	4,000	$ 3,000	14,000	10,000
Proceeds from Stock Options Exercised			79,000	103,000
Share-based Payment Arrangement, Exercise of Option, Tax Benefit			$ 10,000	$ 24,000
Stock Option Plan 2013 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)			5,000	0
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total	$ 616,000		$ 616,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)			2 years 9 months 18 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value			$ 186,000	$ 197,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value			$ 285,000	$ 311,000
Stock Option Plan 2013 [Member] \| Share-based Payment Arrangement, Option [Member]
Common Stock, Capital Shares Reserved for Future Issuance (in shares)	391,835		391,835
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares)	109,100		109,100
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)			10 years

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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