Note 4 - Stock-based Compensation - Summary of Stock Option Activity (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Outstanding, weighted average exercise price (in dollars per share)	$ 5.14	$ 5.17
Outstanding, number of shares (in shares)	56,400	68,400
Granted, number of shares (in shares)
Cancelled, weighted average exercise price (in dollars per share)	$ 5.14	$ 5.30
Cancelled, number of shares (in shares)	(12,000)	(12,000)
Exercised, weighted average exercise price (in dollars per share)	$ 5.14
Exercised, number of shares (in shares)	(44,400)
Exercised, weighted average remaining contractual term (Year)	3 years 361 days
Exercised, aggregate intrinsic value	$ 155,606
Outstanding, weighted average remaining contractual term (Year)		4 years 171 days
Outstanding, aggregate intrinsic value				$ 70,545
Exercisable, weighted average exercise price (in dollars per share)				$ 6.27
Exercisable, number of shares (in shares)		56,400		56,400
Exercisable, weighted average remaining contractual term (Year)		4 years 171 days
Exercisable, aggregate intrinsic value				$ 70,545
Cancelled, weighted average exercise price (in dollars per share)	$ (5.14)	$ (5.30)
Outstanding, weighted average exercise price (in dollars per share)		$ 6.27
Outstanding, number of shares (in shares)	56,400	68,400		56,400
Exercisable, number of shares (in shares)		56,400