Note 16 - Income Taxes (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense (Benefit)		$ 0	$ 0	$ 0
Deferred Tax Liabilities, Net		0	0
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount		(3,000,000)	2,800,000
Unrecognized Tax Benefits that Would Impact Effective Tax Rate		$ 511,000	$ 398,000
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		35.00%
Changes in Deferred Tax Assets Before Valuation Allowance, Due to Changes in Enacted Tax Rate		$ (8,200,000)
Deferred Tax Assets Remeasurement [Member]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount		(8,200,000)
Scenario, Forecast [Member]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%
Domestic Tax Authority [Member] | Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards		57,600,000
Domestic Tax Authority [Member] | Internal Revenue Service (IRS) [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward, Amount		1,600,000
State and Local Jurisdiction [Member]
Operating Loss Carryforwards		47,700,000
State and Local Jurisdiction [Member] | Research Tax Credit Carryforward [Member]
Tax Credit Carryforward, Amount		$ 1,300,000