Note 10 - Stockholders' Equity - Stock Option Plan (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Beginning balance, number outstanding (in shares)	432,930
Exercised during period (in shares)	0	0
Ending balance, number outstanding (in shares)	417,000	432,930
Number of shares exercisable (in shares)	377,000	307,930
The 2001 Non-qualified Stock Option Plan [Member]
Beginning balance, number outstanding (in shares)	22,930	22,930
Granted during period (in shares)
Exercised during period (in shares)
Canceled during period (in shares)	15,930
Ending balance, number outstanding (in shares)	7,000	22,930
Number of shares exercisable (in shares)	7,000	22,930
Beginning balance, weighted average exercise price (in dollars per share)	$ 5.36	$ 5.36
Granted during period, weighted average exercise price (in dollars per share)
Exercised during period, weighted average exercise price (in dollars per share)
Canceled during period, weighted average exercise price (in dollars per share)	4.25
Ending balance, weighted average exercise price (in dollars per share)	7.90	5.36
Number of options exercisable, weighted average exercise price (in dollars per share)	$ 7.90	$ 5.36
Canceled during period (in shares)	(15,930)
The 2007 Share Incentive Plan [Member]
Beginning balance, number outstanding (in shares)	345,000	365,000
Granted during period (in shares)
Exercised during period (in shares)
Canceled during period (in shares)		20,000
Ending balance, number outstanding (in shares)	345,000	345,000
Number of shares exercisable (in shares)	305,000	285,000
Beginning balance, weighted average exercise price (in dollars per share)	$ 12.33	$ 12.35
Granted during period, weighted average exercise price (in dollars per share)
Exercised during period, weighted average exercise price (in dollars per share)
Canceled during period, weighted average exercise price (in dollars per share)		11.61
Ending balance, weighted average exercise price (in dollars per share)	12.33	12.33
Number of options exercisable, weighted average exercise price (in dollars per share)	$ 12.60	$ 12.76
Canceled during period (in shares)		(20,000)
The 2016 Share Incentive Plan [Member]
Beginning balance, number outstanding (in shares)	65,000	20,000
Granted during period (in shares)		60,000
Exercised during period (in shares)
Canceled during period (in shares)		15,000
Ending balance, number outstanding (in shares)	65,000	65,000
Number of shares exercisable (in shares)
Beginning balance, weighted average exercise price (in dollars per share)	$ 5.94
Granted during period, weighted average exercise price (in dollars per share)		5
Exercised during period, weighted average exercise price (in dollars per share)
Canceled during period, weighted average exercise price (in dollars per share)		5
Ending balance, weighted average exercise price (in dollars per share)	5.94	5.94
Number of options exercisable, weighted average exercise price (in dollars per share)
Canceled during period (in shares)		(15,000)