Note 2 - Significant Accounting Policies - Potentially Dilutive Securities Not Included in Calculation Of Diluted Net Loss Per Share (Details) - shares shares in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Anti-dilutive securities not included in calculation of diluted net loss per share (in shares)	8,765,991	9,533,446
Share-based Payment Arrangement, Option [Member]
Anti-dilutive securities not included in calculation of diluted net loss per share (in shares)	4,757,783	5,179,581
Warrant [Member]
Anti-dilutive securities not included in calculation of diluted net loss per share (in shares)	2,116,250	2,116,250
Unvested Restricted Stock Units [Member]
Anti-dilutive securities not included in calculation of diluted net loss per share (in shares)	1,891,958	2,237,615