Note 8 - Stock Based Awards - Activity Under Stock Option Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2021
Balances, number of shares (in shares)	2,173,776
Balances, weighted average exercise price (in dollars per share)	$ 9.59
Options granted, number of shares (in shares)	865,674
Options granted, weighted average exercise price (in dollars per share)	$ 5.55
Options cancelled, number of shares (in shares)	(457,323)
Options cancelled, weighted average exercise price (in dollars per share)	$ 34.53
Options exercised, number of shares (in shares)	(148,452)
Options exercised, weighted average exercise price (in dollars per share)	$ 1.91
Balances, number of shares (in shares)	2,433,675
Balances, weighted average exercise price (in dollars per share)	$ 3.93
Balances, weighted average remaining contractual life (Year)	7 years
Balances, aggregate intrinsic value	$ 3,269
Outstanding and expected to vest, number of shares (in shares)	2,278,219
Outstanding and expected to vest, weighted average exercise price (in dollars per share)	$ 3.77
Outstanding and expected to vest, weighted average remaining contractual life (Year)	6 years 10 months 24 days
Outstanding and expected to vest, aggregate intrinsic value	$ 3,269
Exercisable, number of shares (in shares)	1,473,616
Exercisable, weighted average exercise price (in dollars per share)	$ 2.44
Exercisable, weighted average remaining contractual life (Year)	5 years 7 months 6 days
Exercisable, aggregate intrinsic value	$ 3,269