Note 18 - Quarterly Financial Data (Unaudited) - Quarterly Financial Information (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
Revenue	$ 3,916	$ 454	$ 85	$ 5,407	$ 1,767	$ 281	$ 608	$ 1,503	$ 9,862	$ 4,159
Operating expenses	9,542	5,168	5,082	5,855	5,953	6,161	8,383	5,418	25,647	25,915
Net loss	$ (6,409)	$ (5,260)	$ (5,637)	$ (1,339)	(4,902)	(6,548)	(8,871)	2,314	$ (18,645)	$ (18,007)
Net loss per share – basic and diluted (in dollars per share)	$ (0.13)	$ (0.32)	$ (0.39)	$ (0.18)					$ (0.86)	$ (2.90)
Net loss attributable to common stockholders					$ (4,902)	$ (6,548)	$ (8,871)	$ 1,975	$ (18,645)	$ (18,346)
Net income (loss) per share – basic (in dollars per share)					$ (0.69)	$ (0.92)	$ (1.24)	$ 0.54
Net income (loss) per share – diluted (in dollars per share)					$ (0.69)	$ (0.92)	$ (1.24)	$ 0.49