Note 11 - Stock-based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Stock-based compensation	$ 172	$ 263
Stock Options Issued to Employees and Board Members [Member] | Research and Development Expense [Member]
Stock-based compensation	76	28
Stock Options Issued to Employees and Board Members [Member] | Selling, General and Administrative Expenses [Member]
Stock-based compensation	260	206
Restricted Stock [Member] | Research and Development Expense [Member]
Stock-based compensation	(79)	16
Restricted Stock [Member] | Selling, General and Administrative Expenses [Member]
Stock-based compensation	$ (85)	$ 13