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Note 1 - Organization and Operations (Details Textual) - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended 12 Months Ended
Aug. 02, 2017
Nov. 12, 2015
Jun. 30, 2018
Dec. 31, 2017
Cash, Cash Equivalents, and Short-term Investments, Total     $ 26,500  
Retained Earnings (Accumulated Deficit), Ending Balance     $ (200,701) $ (189,741)
Preferred Stock, Par or Stated Value Per Share     $ 0.001 $ 0.001
Series A Convertible Preferred Stock [Member]        
Stock Issued During Period, Shares, New Issues 22,000      
Payments of Stock Issuance Costs $ 500      
Preferred Stock, Par or Stated Value Per Share $ 0.001   $ 0.001 $ 0.001
Shares Issued, Price Per Share $ 1,000      
Proceeds from Issuance of Convertible Preferred Stock $ 22,000      
Sales Agreement [Member] | Cowen and Company, LLC [Member] | Common Stock [Member]        
Sale of Stock, Maximum Value of Stock Offered   $ 40,000    
Sale of Stock, Commission to Sales Agent, Percentage     3.00%  
Stock Issued During Period, Shares, New Issues     0 896,811
Proceeds from Issuance of Common Stock       $ 1,400
Payments of Stock Issuance Costs       $ 100