Note 11 - Stock-based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Stock-based compensation	$ 501	$ 107	$ 673	$ 370
Stock Options Issued to Employees and Board Members [Member] | Research and Development Expense [Member]
Stock-based compensation	135	33	211	61
Stock Options Issued to Employees and Board Members [Member] | Selling, General and Administrative Expenses [Member]
Stock-based compensation	362	139	622	345
Restricted Stock [Member] | Research and Development Expense [Member]
Stock-based compensation	2	(34)	(77)	(18)
Restricted Stock [Member] | Selling, General and Administrative Expenses [Member]
Stock-based compensation	$ 2	$ (31)	$ (83)	$ (18)