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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer, Including Assessed Tax	$ 15,126	$ 5,613	$ 9,966	$ 4,281	$ 3,238	$ 5,317	$ 3,878	$ 3,189	$ 34,986	$ 15,622	$ 11,698
Operating expenses	25,841	23,729	29,059	22,516	17,790	30,365	18,216	15,607
Operating loss	(10,715)	(18,116)	(19,093)	(18,235)	(14,552)	(25,048)	(14,338)	(12,418)	$ (66,159)	$ (66,356)	$ (58,400)
Net loss	$ (11,275)	$ (18,842)	$ (19,700)	$ (18,735)	$ (14,704)	$ (24,357)	$ (15,172)	$ (13,424)
Net loss per share - basic and diluted (in dollars per share)	$ (0.13)	$ (0.22)	$ (0.23)	$ (0.22)	$ (0.17)	$ (0.30)	$ (0.20)	$ (0.19)	$ (0.80)	$ (0.87)	$ (0.73)