Note 6 - Stockholders' Equity and Stock-based Compensation - Summary of Time-based Option Activity (Details) - Time-based Options [Member] - $ / shares	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Options balances, number of shares (in shares)	4,796,716
Options balances, weighted average exercise price (in dollars per share)	$ 16.44
Options balances, weighted average remaining life (Year)	6 years 1 month 28 days	7 years 18 days
Options granted, number of shares (in shares)	1,240,100
Options granted, weighted average exercise price (in dollars per share)	$ 2.59
Options exercised, number of shares (in shares)	0
Options exercised, weighted average exercise price (in dollars per share)	$ 0
Options canceled, number of shares (in shares)	(674,465)
Options canceled, weighted average exercise price (in dollars per share)	$ 16.73
Options expired, number of shares (in shares)	(631,957)
Options expired, weighted average exercise price (in dollars per share)	$ 15.05
Options balances, number of shares (in shares)	4,730,394	4,796,716
Options balances, weighted average exercise price (in dollars per share)	$ 12.95	$ 16.44
Options exercisable, number of shares (in shares)	3,230,670
Options exercisable, weighted average exercise price (in dollars per share)	$ 15.39
Options exercisable, weighted average remaining life (Year)	5 years 25 days