Note 4 - Stock-based Compensation (Details Textual) - USD ($)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance (in shares)	22,667		17,500
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)	10,000	10,000
Share-based Payment Arrangement, Expense	$ 204,600	$ 10,100
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total	$ 597,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year)	4 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period (in shares)	3,833	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value	$ 39,800
Proceeds from Stock Options Exercised	33,647	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	152,800
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	44,900
Approximation [Member]
Proceeds from Stock Options Exercised	$ 33,600
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)	36,550	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares)	2,700
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share)	$ 11.01
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)	4 years
The 1995 Incentive Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares)	442,750
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance (in shares)	1,000
The 2019 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares)	4,257
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period (in shares)	12,750
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross (in shares)	23,250
The 2019 Equity Incentive Plan [Member] | Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares)				102,157
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period (in shares)	1,000
The 2019 Equity Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares)	90,850
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period (in shares)	2,700