Note 5 - Share-Based Compensation - Summary of Stock Option Activity (Details) - Employee and Nonemployee Stock Option [Member] - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2019
Outstanding, shares under options (in shares)	134,043
Outstanding, weighted average exercise price (in dollars per share)	$ 20.46
Granted, shares under options (in shares)	16,536
Granted, weighted average exercise price (in dollars per share)	$ 38.61
Exercised, shares under options (in shares)	(31,058)
Exercised, weighted average exercise price (in dollars per share)	$ 14.18
Outstanding, shares under options (in shares)	119,521
Outstanding, weighted average exercise price (in dollars per share)	$ 24.61
Outstanding, weighted average remaining contractual life (Year)	5 years 248 days
Outstanding, aggregate intrinsic value	$ 2,032
Exercisable, shares under options (in shares)	95,485
Exercisable, weighted average exercise price (in dollars per share)	$ 21.86
Exercisable, weighted average remaining contractual life (Year)	5 years 43 days
Exercisable, aggregate intrinsic value	$ 1,886
Vested and expected to vest, shares under options (in shares)	119,521
Vested and expected to vest, weighted average exercise price (in dollars per share)	$ 24.61
Vested and expected to vest, weighted average remaining contractual life (Year)	5 years 248 days
Vested and expected to vest, aggregate intrinsic value	$ 2,032