Note 9 - Income Taxes - Differences Between the Company's Effective Tax Rate and the Statutory Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income tax benefit at federal statutory rate (21% for 2020, 2019 and 2018)	$ (38,391)	$ (22,868)	$ (21,263)
State and local income taxes net of federal tax benefit	(2,544)	(1,591)	(2,547)
Permanent items	774	691	503
Rate change	(82)	625	(29)
Expiration of attribute carryforwards	3,774	3,976	2,183
Research and development tax credits	(4,080)	(4,938)	(4,905)
Foreign rate differential	542
Other	1,456	281	18
Change in valuation allowance	38,551	23,824	26,040
Income tax expense	$ 0	$ 0	$ 0