Note 11 - Stock-based Compensation - Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Stock-based compensation	$ 925	$ 172
Stock Options Issued to Employees and Board Members [Member] | Research and Development Expense [Member]
Stock-based compensation	125	76
Stock Options Issued to Employees and Board Members [Member] | Selling, General and Administrative Expenses [Member]
Stock-based compensation	437	260
Restricted Stock [Member] | Research and Development Expense [Member]
Stock-based compensation	348	(79)
Restricted Stock [Member] | Selling, General and Administrative Expenses [Member]
Stock-based compensation	$ 15	$ (85)