Note 7 - Stock-based Compensation - Stock Option Activity (Details) - Stock Option Plan 2013 [Member] - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 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 exercised, shares (in shares)	(12,050)
Options exercised, weighted average exercise price (in dollars per share)	$ 6.68
Options outstanding, shares (in shares)	295,335
Options outstanding, weighted average exercise price (in dollars per share)	$ 18.33
Options outstanding, weighted average remaining contractual term (Year)	6 years
Options outstanding, intrinsic value	$ 899,000
Options exercisable, shares (in shares)	122,785
Options exercisable, weighted average exercise price (in dollars per share)	$ 12.89
Options exercisable, weighted average remaining contractual term (Year)	4 years 73 days
Options exercisable, intrinsic value	$ 899,000
Expected to vest, shares (in shares)	152,897
Expected to vest, weighted average exercise price (in dollars per share)	$ 22.20
Expected to vest, weighted average remaining contractual term (Year)	7 years 73 days