Note 11 - Stock-based Compensation - Allocation of Recognized Period Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total compensation expense recognized	$ 4,392	$ 5,209	$ 4,142
Research and Development Expense [Member]
Total compensation expense recognized	1,770	1,760	1,371
Selling, General and Administrative Expenses [Member]
Total compensation expense recognized	$ 2,622	$ 3,449	$ 2,771