Note 9 - Stock-based Compensation Expense - Outstanding and Exercisable Options (Details)	6 Months Ended
	Jun. 30, 2021 USD ($) $ / shares shares
Exercise Price Range 1 [Member]
Exercise price range, lower limit (in dollars per share)	$ 4.00
Exercise price range, upper limit (in dollars per share)	$ 7.00
Number of options outstanding (in shares) | shares	195,000
Number of options outstanding, weighted average remaining contractual term (Year)	9 years 9 months 18 days
Number of options outstanding, weighted average exercise price (in dollars per share)	$ 4.43
Number of options outstanding, intrinsic value | $	$ 36,000
Number of options exercisable (in shares) | shares	45,000
Number of options exercisable, weighted average exercise price (in dollars per share)	$ 5.00
Number of options exercisable, intrinsic value | $	$ 0
Exercise Price Range 2 [Member]
Exercise price range, lower limit (in dollars per share)	$ 7.01
Exercise price range, upper limit (in dollars per share)	$ 10.00
Number of options outstanding (in shares) | shares	20,000
Number of options outstanding, weighted average remaining contractual term (Year)	7 years
Number of options outstanding, weighted average exercise price (in dollars per share)	$ 8.07
Number of options outstanding, intrinsic value | $	$ 0
Number of options exercisable (in shares) | shares	20,000
Number of options exercisable, weighted average exercise price (in dollars per share)	$ 8.07
Number of options exercisable, intrinsic value | $	$ 0
Exercise Price Range 3 [Member]
Exercise price range, lower limit (in dollars per share)	$ 10.01
Exercise price range, upper limit (in dollars per share)	$ 12.00
Number of options outstanding (in shares) | shares	220,000
Number of options outstanding, weighted average remaining contractual term (Year)	3 years 8 months 12 days
Number of options outstanding, weighted average exercise price (in dollars per share)	$ 10.81
Number of options outstanding, intrinsic value | $	$ 0
Number of options exercisable (in shares) | shares	200,000
Number of options exercisable, weighted average exercise price (in dollars per share)	$ 10.87
Number of options exercisable, intrinsic value | $	$ 0
Exercise Price Range 4 [Member]
Exercise price range, lower limit (in dollars per share)	$ 12.01
Exercise price range, upper limit (in dollars per share)	$ 15.00
Number of options outstanding (in shares) | shares	125,000
Number of options outstanding, weighted average remaining contractual term (Year)	1 year
Number of options outstanding, weighted average exercise price (in dollars per share)	$ 15.00
Number of options outstanding, intrinsic value | $	$ 0
Number of options exercisable (in shares) | shares	125,000
Number of options exercisable, weighted average exercise price (in dollars per share)	$ 15.00
Number of options exercisable, intrinsic value | $	$ 0