Note 8 - Stock-based Compensation Expense - Stock Option Plan (Details) - $ / shares	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Granted during period (in shares)	12,500
Exercised during period (in shares)	0	0
Ending balance, number outstanding (in shares)	628,500
Number of shares exercisable (in shares)	285,000
The 2007 Share Incentive Plan [Member]
Beginning balance, number outstanding (in shares)	220,000
Granted during period (in shares)	0
Exercised during period (in shares)	0
Canceled during period (in shares)	0
Ending balance, number outstanding (in shares)	220,000
Number of shares exercisable (in shares)	220,000
Beginning balance, weighted average exercise price (in dollars per share)	$ 12.56
Granted during period, weighted average exercise price (in dollars per share)	0
Exercised during period, weighted average exercise price (in dollars per share)	0
Canceled during period, weighted average exercise price (in dollars per share)	0
Ending balance, weighted average exercise price (in dollars per share)	12.56
Number of options exercisable, weighted average exercise price (in dollars per share)	$ 12.56
The 2016 Share Incentive Plan [Member]
Beginning balance, number outstanding (in shares)	398,500
Granted during period (in shares)	12,500
Exercised during period (in shares)	0
Canceled during period (in shares)	(2,500)
Ending balance, number outstanding (in shares)	408,500
Number of shares exercisable (in shares)	65,000
Beginning balance, weighted average exercise price (in dollars per share)	$ 4.43
Granted during period, weighted average exercise price (in dollars per share)	4.60
Exercised during period, weighted average exercise price (in dollars per share)	0
Canceled during period, weighted average exercise price (in dollars per share)	4.01
Ending balance, weighted average exercise price (in dollars per share)	4.43
Number of options exercisable, weighted average exercise price (in dollars per share)	$ 5.94