Note 10 - Income Taxes (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Income Tax Expense (Benefit)	$ 0	$ 0
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%
Effective Income Tax Rate Reconciliation, Percent	0.00%	0.00%
Unrecognized Tax Benefits	$ 0	$ 0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	0	0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	$ 0	$ 0
Open Tax Year	2016 2017 2018 2019 2020 2021 2022 2023
Research Tax Credit Carryforward [Member]
Tax Credit Carryforward, Amount	$ 5,600
Tax Credit Carryforward, Expiration Date	Dec. 31, 2034
Domestic Tax Authority [Member]
Operating Loss Carryforwards	$ 147,100
Operating Loss Carryforwards Not Subject to Expiration	113,700
Operating Loss Carryforwards Subject to Expiration	$ 33,400
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2034
State and Local Jurisdiction [Member]
Operating Loss Carryforwards	$ 96,700
Operating Loss Carryforwards Not Subject to Expiration	83,700
Operating Loss Carryforwards Subject to Expiration	$ 12,900
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2032