Note 6 - Liabilities Related to Sale of Future Royalties - Deferred Revenue Activity (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Current portion	$ (3,150)	$ (3,328)
Long-term portion	12,519	$ 14,413
HealthCare Royalty Partners III, L.P. [Member] | Royalty Arrangement [Member]
Total liability related to sale of future royalties, start of period	17,741
Non-cash royalty revenue paid to HCRP	(3,132)
Non-cash interest expense recognized	1,060
Total liability related to sale of future royalties, end of period	15,669
Current portion	(3,150)
Long-term portion	$ 12,519