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Note 10 - Income Taxes (Details Textual) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Expense (Benefit), Total $ 0 $ 0 $ 0  
Deferred Tax Assets, Tax Credit Carryforwards, Research $ 35,670 $ 25,919    
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 35.00%    
Adjustment of Deferred Tax (Asset) Liability, Revaluation Based On New Tax Rate   $ 74,300    
Unrecognized Tax Benefits, Ending Balance $ 2,385 120 $ 120 $ 120
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total 0 0    
Deferred Tax Assets Revaluated [Member]        
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount   $ (74,300)    
Domestic Tax Authority [Member]        
Operating Loss Carryforwards, Total 715,300      
Expiring Operating Loss Carryforwards 549,700      
Indefinite Operating Loss Carryforwards 165,600      
Deferred Tax Assets, Tax Credit Carryforwards, Research 27,200      
State and Local Jurisdiction [Member]        
Deferred Tax Assets, Tax Credit Carryforwards, Research 13,300      
State and Local Jurisdiction [Member] | California Franchise Tax Board [Member]        
Operating Loss Carryforwards, Total 122,300      
State and Local Jurisdiction [Member] | Other State Jurisdiction [Member]        
Operating Loss Carryforwards, Total $ 824,900