Note 8 - Income Taxes (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued		$ 97,298
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		34.00%	34.00%	34.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability		$ 7,300,000
Undistributed Earnings of Foreign Subsidiaries		0
Scenario, Forecast [Member]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%
Federal and US State Except California [Member]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount		(26,300,000)
Domestic Tax Authority [Member]
Operating Loss Carryforwards		34,700,000
Tax Credit Carryforward, Amount		6,109,000
Domestic Tax Authority [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward, Amount		$ 6,100,000
Domestic Tax Authority [Member] | Earliest Tax Year [Member]
Operating Loss Carryforwards Expiration Year 1		2029
Tax Credit Carryforward Expiration Year		2024
Domestic Tax Authority [Member] | Earliest Tax Year [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward Expiration Year		2024
Domestic Tax Authority [Member] | Latest Tax Year [Member]
Operating Loss Carryforwards Expiration Year 1		2035
Tax Credit Carryforward Expiration Year		2036
Domestic Tax Authority [Member] | Realizable Deferred Tax Assets due to Achievement of Three Years Cumulative Income [Member]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount		$ (26,300,000)
State and Local Jurisdiction [Member]
Operating Loss Carryforwards		20,773,000
Tax Credit Carryforward, Amount		6,872,000
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member] | California Franchise Tax Board [Member]
Tax Credit Carryforward, Amount		6,600,000
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]
Operating Loss Carryforwards Expiration Year 1		2029
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
State and Local Jurisdiction [Member] | Latest Tax Year [Member]
Operating Loss Carryforwards Expiration Year 1		2035