Note 7 - Stock-based Compensation - Stock Option Activity (Details) - Stock Option Plan 2013 [Member] - USD ($)	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Options outstanding, shares (in shares)	302,385
Options outstanding, weighted average exercise price (in dollars per share)	$ 17.73
Options granted, shares (in shares)	5,000	0
Options granted, weighted average exercise price (in dollars per share)	$ 26.42
Options cancelled, shares (in shares)	(7,600)
Options cancelled, weighted average exercise price (in dollars per share)	$ 24.40
Options exercised, shares (in shares)	(17,050)
Options exercised, weighted average exercise price (in dollars per share)	$ 7.29
Options outstanding, shares (in shares)	282,735
Options outstanding, weighted average exercise price (in dollars per share)	$ 18.34
Options outstanding, weighted average remaining contractual term (Year)	5 years 6 months
Options outstanding, intrinsic value	$ 947,347
Options exercisable, shares (in shares)	114,185
Options exercisable, weighted average exercise price (in dollars per share)	$ 12.71
Options exercisable, weighted average remaining contractual term (Year)	3 years 7 months 6 days
Options exercisable, intrinsic value	$ 947,347
Expected to vest, shares (in shares)	150,431
Expected to vest, weighted average exercise price (in dollars per share)	$ 22.15
Expected to vest, weighted average remaining contractual term (Year)	6 years 9 months 18 days
Options expected to vest, intrinsic value	$ 0