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Note 1 - Summary of Significant Accounting Policies (Details Textual)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jan. 01, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Cash and Cash Equivalents, at Carrying Value, Ending Balance $ 87,490 $ 87,490 $ 113,959   $ 90,642 $ 138,909
Cash and Cash Equivalents, Period Increase (Decrease), Total   $ (3,100)        
Number of Years Cash is Sufficient to Fund Operations   1 year        
Proceeds from Issuance of Common Stock 14,500 $ 24,815      
Cumulative Effect of New Accounting Principle in Period of Adoption 35 35        
Accounts Receivable, Net, Current, Total 3,843 3,843     3,972  
Restricted Cash and Cash Equivalents, Noncurrent, Total 1,526 1,526 $ 2,003   1,522 $ 2,001
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment, Ending Balance 2,198 $ 2,198     $ 1,716  
Number of Operating Segments   1        
Retained Earnings [Member]            
Cumulative Effect of New Accounting Principle in Period of Adoption $ 35 $ 35        
Accounting Standards Update 2014-09 [Member]            
Accounts Receivable, Net, Current, Total       $ 35    
Accounting Standards Update 2014-09 [Member] | Retained Earnings [Member]            
Cumulative Effect of New Accounting Principle in Period of Adoption       $ 35