Note 7 - Stock-based Compensation - Summary of Stock Option Activity (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Outstanding (in shares)	417,000	450,000	450,000
Outstanding, grant date weighted-average exercise price (in dollars per share)	$ 2.79	$ 2.78	$ 2.78
Outstanding, weighted average remaining contractual life (Year)		1 year 10 months 9 days	1 year 7 months 9 days	2 years 5 months 8 days
Outstanding, intrinsic value		$ 0	$ 2,000	$ 27,000
Granted (in shares)	6,000	10,000
Granted, grant date weighted-average exercise price (in dollars per share)	$ 2.92	$ 1.88
Granted, weighted average remaining contractual life (Year)	7 years	7 years
Exercised (in shares)	(22,000)
Exercised, grant date weighted-average exercise price (in dollars per share)	$ 2.65
Forfeited (in shares)	(7,000)	(40,000)
Forfeited, grant date weighted-average exercise price (in dollars per share)	$ 2.51	$ 2.58
Exercisable (in shares)	383,000	403,000
Exercisable, grant date weighted-average exercise price (in dollars per share)	$ 2.80	$ 2.80
Exercisable, weighted average remaining contractual life (Year)		1 year 8 months 8 days
Outstanding (in shares)	394,000	420,000	417,000	450,000
Outstanding, grant date weighted-average exercise price (in dollars per share)	$ 2.80	$ 2.78	$ 2.79	$ 2.78
Minimum [Member]
Outstanding, weighted average remaining contractual life (Year)	10 months 24 days
Exercisable, weighted average remaining contractual life (Year)	9 months 7 days
Maximum [Member]
Outstanding, intrinsic value	$ 17,000