Note 15 - Unaudited Quarterly Results (Unaudited) - Summarized Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from Contract with Customer, Including Assessed Tax	$ 3,238	$ 5,317	$ 3,878	$ 3,189	$ 3,889	$ 2,697	$ 2,765	$ 2,347	$ 15,622	$ 11,698	$ 69,429
Operating expenses	17,790	30,365	18,216	15,607	17,171	17,002	18,325	17,600	81,978	70,098	56,325
Operating loss	(14,552)	(25,048)	(14,338)	(12,418)	(13,282)	(14,305)	(15,560)	(15,253)	(66,356)	(58,400)	13,104
Net (loss) income	$ (14,704)	$ (24,357)	$ (15,172)	$ (13,424)	$ (2,665)	$ (15,352)	$ (16,636)	$ (16,360)	$ (67,657)	$ (51,013)	$ 10,733
Net loss per share - basic and diluted (in dollars per share)	$ (0.17)	$ (0.30)	$ (0.20)	$ (0.19)	$ (0.04)	$ (0.22)	$ (0.24)	$ (0.23)	$ (0.87)	$ (0.73)