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Note 8 - Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2017
Notes Tables  
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]
    2017   2016   2015
Income tax benefit at federal statutory rate   $
(23,024
 )   $
(19,300
 )   $
(15,057
 )
State and local income taxes net of federal tax benefit    
(1,611
 )    
(1,173
 )    
(819
 )
Permanent items    
910
      
1,057
      
560
  
Rate change    
71,155
      
1,080
      
1,012
  
Expiration of attribute carryforwards    
918
      
559
      
330
  
Effect of ASU 2016-09    
(5,949
 )    
—
      
—
  
Research and development tax credits    
(1,977
 )    
(4,681
 )    
(10,454
 )
Orphan drug credit    
564
      
1,798
      
4,307
  
Other    
1,639
      
822
      
(218
 )
Change in valuation allowance    
(42,625
 )    
19,838
      
20,339
  
Income tax expense   $
—
     $
—
     $
—
  
Schedule of Unrecognized Tax Benefits Roll Forward [Table Text Block]
    2017   2016
Balance at January 1,   $
4,255
     $
3,085
  
Additions to current period tax positions    
495
      
1,170
  
Additions to prior period tax positions    
—
      
—
  
Reductions to prior period tax provisions    
—
      
—
  
Balance at December 31,   $
4,750
     $
4,255
  
Schedule of Deferred Tax Assets and Liabilities [Table Text Block]
    2017   2016
Deferred tax assets:                
Net federal and state operating losses   $
117,787
     $
158,618
  
Research and development credits    
55,208
      
53,231
  
Deferred revenue    
1,854
      
3,484
  
Stock-based compensation    
6,424
      
8,952
  
Other    
2,046
      
2,849
  
Total deferred tax assets    
183,319
      
227,134
  
Deferred tax liabilities:                
Fixed assets    
(421
 )    
(674
 )
Foreign currency derivative    
(478
 )    
(1,415
 )
Total deferred tax liabilities    
(899
 )    
(2,089
 )
                 
Valuation allowance    
(182,420
 )    
(225,045
 )
Net deferred tax assets   $
—
     $
—