Note 11 - Income Taxes (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Expense (Benefit), Total
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	4,700,000	7,600,000
Deferred Tax Liability, Operating Lease Right of Use Asset	1,355,000
Deferred Tax Assets, Operating Lease Liability	1,237,000
Change in Deferred Tax Asset due to New Accounting Principle in Period of Adoption		(6,000)
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total	0	0
State and Local Jurisdiction [Member]
Income Tax Expense (Benefit), Total		$ 0
Operating Loss Carryforwards, Total	216,100,000
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward, Amount	3,600,000
Domestic Tax Authority [Member]
Operating Loss Carryforwards, Total	301,000,000
Operating Loss Carryforwards, Not Subject to Expiration	43,500,000
Domestic Tax Authority [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward, Amount	$ 3,400,000