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Note 8 - Financial Highlights (Tables)
 9 Months Ended
Sep. 30, 2018
Notes Tables  
Financial Highlights [Table Text Block]
   
Three Months Ended
September 30,
    
Nine Months Ended
September 30,
  
   
2018
    
2017
    
2018
    
2017
  
Net asset value per Share, beginning of period
   $
15.08
     $
15.58
     $
15.91
     $
15.40
  
                                 
Net investment loss
(a)
    
(0.02
 )    
(0.02
 )    
(0.06
 )    
(0.06
 )
Net realized and unrealized gain (loss)
(b)
    
(1.62
 )    
0.36
      
(2.41
 )    
0.58
  
Net increase (decrease) in net assets from operations
    
(1.64
 )    
0.34
      
(2.47
 )    
0.52
  
Net asset value per Share, end of period
   $
13.44
     $
15.92
     $
13.44
     $
15.92
  
                                 
Total return, at net asset value
(c)(d)
    
(10.88
%)
    
2.18
%
    
(15.52
%)
    
3.38
%
                                 
Ratio to average net assets:                                
Net investment loss
(e)
    
(0.50
%)
    
(0.50
%)
    
(0.50
%)
    
(0.50
%)
Expenses
(e)
    
0.50
%
    
0.50
%
    
0.50
%
    
0.50
%