Note 11 - Stock-based Compensation - Allocation of Recognized Period Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Total compensation expense recognized	$ 1,586	$ 960	$ 2,189	$ 1,536
Research and Development Expense [Member]
Total compensation expense recognized	326	174	561	449
General and Administrative Expense [Member]
Total compensation expense recognized	$ 1,260	$ 786	$ 1,628	$ 1,087