Note 11 - Stock-based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Stock-based compensation	$ 674	$ 568	$ 1,347	$ 938
Stock Options Issued to Employees and Board Members [Member] | Research and Development Expense [Member]
Stock-based compensation	142	68	353	129
Stock Options Issued to Employees and Board Members [Member] | Selling, General and Administrative Expenses [Member]
Stock-based compensation	500	236	1,122	581
Restricted Stock [Member] | Research and Development Expense [Member]
Stock-based compensation	16	132	(61)	114
Restricted Stock [Member] | Selling, General and Administrative Expenses [Member]
Stock-based compensation	$ 16	$ 132	$ (67)	$ 114