Note 8 - Stock Based Awards - Activity Under Stock Option Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 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)	608,950
Options granted, weighted average exercise price (in dollars per share)	$ 6.02
Options cancelled, number of shares (in shares)	(343,537)
Options cancelled, weighted average exercise price (in dollars per share)	$ 43.89
Options exercised, number of shares (in shares)	(140,952)
Options exercised, weighted average exercise price (in dollars per share)	$ 1.98
Balances, number of shares (in shares)	2,298,237
Balances, weighted average exercise price (in dollars per share)	$ 3.99
Balances, weighted average remaining contractual life (Year)	7 years 1 month 6 days
Balances, aggregate intrinsic value	$ 5,905
Outstanding and expected to vest, number of shares (in shares)	2,125,891
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	$ 5,871
Exercisable, number of shares (in shares)	1,373,881
Exercisable, weighted average exercise price (in dollars per share)	$ 2.12
Exercisable, weighted average remaining contractual life (Year)	5 years 7 months 6 days
Exercisable, aggregate intrinsic value	$ 5,707