Note 7 - Income Taxes (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 400,000	$ 7,300,000
Open Tax Year	2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total	$ 107,000
Federal and US State Except California [Member]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount		$ (26,300,000)
Domestic Tax Authority [Member]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	16,900,000
Operating Loss Carryforwards, Total	45,700,000
Operating Loss Carryforwards Without Expiration	11,800,000
Domestic Tax Authority [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward, Amount	$ 6,700,000
Domestic Tax Authority [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards Expiration Year 1	2029
Domestic Tax Authority [Member] | Earliest Tax Year [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward Expiration Year	2024
State and Local Jurisdiction [Member]
Operating Loss Carryforwards, Total	$ 20,500,000
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member] | California Franchise Tax Board [Member]
Tax Credit Carryforward, Amount	7,400,000
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member] | Massachusetts Department of Revenue [Member]
Tax Credit Carryforward, Amount	$ 300,000
State and Local Jurisdiction [Member] | Earliest Tax Year [Member] | Research Tax Credit Carryforward [Member] | Massachusetts Department of Revenue [Member]
Tax Credit Carryforward Expiration Year	2021