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Note 8 - Financial Highlights (Tables)
 12 Months Ended
Dec. 31, 2017
Notes Tables  
Financial Highlights [Table Text Block]
   
December 31,
  
   
2017
    
2016
    
2015
  
Net asset value per Share, beginning of year
   $
11.16
     $
10.25
     $
11.61
  
                         
Net investment loss
(a)
    
(0.03
 )    
(0.03
 )    
(0.03
 )
Net realized and unrealized gain (loss)
(b)
    
1.32
      
0.94
      
(1.33
 )
Net increase (decrease) in net assets from operations
    
1.29
      
0.91
      
(1.36
 )
Net asset value per Share, end of year
   $
12.45
     $
11.16
     $
10.25
  
                         
Total return, at net asset value
(c)
    
11.56
%
    
8.88
%
    
(11.71
)%
                         
Ratio to average net assets:
                        
Net investment loss
    
(0.25
)%
    
(0.25
)%
    
(0.25
)%
Expenses
    
0.25
%
    
0.25
%
    
0.25
%