Note 9 - Stock-based Compensation Expense - Stock Option Plan (Details)	6 Months Ended
	Jun. 30, 2021 $ / shares shares
Granted during period (in shares)	150,000
Exercised during period (in shares)	0
Ending balance, number outstanding (in shares)	560,000
Number of shares exercisable (in shares)	390,000
The 2001 Non-qualified Stock Option Plan [Member]
Beginning balance, number outstanding (in shares)	7,000
Granted during period (in shares)	0
Exercised during period (in shares)	0
Canceled during period (in shares)	(7,000)
Ending balance, number outstanding (in shares)	0
Number of shares exercisable (in shares)	0
Beginning balance, weighted average exercise price (in dollars per share) | $ / shares	$ 7.90
Granted during period, weighted average exercise price (in dollars per share) | $ / shares	0
Exercised during period, weighted average exercise price (in dollars per share) | $ / shares	0
Canceled during period, weighted average exercise price (in dollars per share) | $ / shares	7.90
Ending balance, weighted average exercise price (in dollars per share) | $ / shares	0
Number of options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 0
The 2007 Share Incentive Plan [Member]
Beginning balance, number outstanding (in shares)	345,000
Granted during period (in shares)	0
Exercised during period (in shares)	0
Canceled during period (in shares)	0
Ending balance, number outstanding (in shares)	345,000
Number of shares exercisable (in shares)	325,000
Beginning balance, weighted average exercise price (in dollars per share) | $ / shares	$ 12.33
Granted during period, weighted average exercise price (in dollars per share) | $ / shares	0
Exercised during period, weighted average exercise price (in dollars per share) | $ / shares	0
Canceled during period, weighted average exercise price (in dollars per share) | $ / shares	0
Ending balance, weighted average exercise price (in dollars per share) | $ / shares	12.33
Number of options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 12.46
The 2016 Share Incentive Plan [Member]
Beginning balance, number outstanding (in shares)	65,000
Granted during period (in shares)	150,000
Exercised during period (in shares)	0
Canceled during period (in shares)	0
Ending balance, number outstanding (in shares)	215,000
Number of shares exercisable (in shares)	65,000
Beginning balance, weighted average exercise price (in dollars per share) | $ / shares	$ 5.94
Granted during period, weighted average exercise price (in dollars per share) | $ / shares	4.26
Exercised during period, weighted average exercise price (in dollars per share) | $ / shares	0
Canceled during period, weighted average exercise price (in dollars per share) | $ / shares	0
Ending balance, weighted average exercise price (in dollars per share) | $ / shares	4.76
Number of options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 5.94