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Note 1 - Organization and Operations (Details Textual) - USD ($)
3 Months Ended 12 Months Ended
Aug. 02, 2017
Nov. 12, 2015
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Sep. 30, 2019
Cash, Cash Equivalents, and Short-term Investments, Total       $ 21,900,000   $ 9,300,000
Retained Earnings (Accumulated Deficit), Ending Balance       $ (210,470,000) $ (189,741,000) $ (223,852,000)
Preferred Stock, Par or Stated Value Per Share       $ 0.001 $ 0.001 $ 0.001
Series A Convertible Preferred Stock [Member]            
Stock Issued During Period, Shares, New Issues 22,000          
Payments of Stock Issuance Costs $ 500,000          
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 $ 22,000,000          
Common Stock [Member]            
Stock Issued During Period, Shares, New Issues     1,494,579 1,494,579 896,811  
Sales Agreement [Member] | Cowen and Company, LLC [Member] | Common Stock [Member]            
Sale of Stock, Maximum Value of Stock Offered   $ 40,000,000        
Sale of Stock, Commission to Sales Agent, Percentage       3.00%    
Stock Issued During Period, Shares, New Issues         896,811  
Proceeds from Issuance of Common Stock         $ 1,400,000  
Payments of Stock Issuance Costs         $ 100,000  
Sales Agreement [Member] | New Leaf Venture Partners LLC [Member] | Common Stock [Member]            
Stock Issued During Period, Shares, New Issues       1,494,579    
Proceeds from Issuance of Common Stock       $ 3,000,000    
Payments of Stock Issuance Costs       $ 46,000