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Note 10 - Investment Valuation (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table Text Block]
   
Level 1
    
Level 2
    
Level 3
   
Total
  
December 31, 2018
                                
Futures contracts
(a)
   $ 
(116,523,249
 )    $
—
     $
—
     $
(116,523,249
 )
U.S. Treasury bills    
 —
      
1,199,726,652
      
—
      
1,199,726,652
  
                                 
December
31,
201
7
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Futures contracts
(a)
   $
74,398,485
     $
—
     $
—
     $
74,398,485
  
U.S. Treasury bills    
—
      
1,343,654,642
      
—
      
1,343,654,642