Note 6 - Liabilities Related to Sale of Future Royalties - Deferred Revenue Activity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Current portion	$ (1,822)	$ (2,779)
Long-term portion	13,239	$ 12,150
HealthCare Royalty Partners III, L.P. [Member] | Royalty Arrangement [Member]
Total liability related to sale of future royalties, start of period	14,929
Non-cash royalty revenue paid to HCRP	(334)
Non-cash interest expense recognized	466
Total liability related to sale of future royalties, end of period	15,061
Current portion	(1,822)
Long-term portion	$ 13,239