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Note 10 - Investment Valuation (Tables)
9 Months Ended
Sep. 30, 2015
Fair Value Disclosures [Abstract]  
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Table Text Block]
   

Level 1

   

Level 2

   

Level 3

   

Total

 

September 30, 2015

                               

Futures contracts(a)

  $ (16,884,863 )   $     $     $ (16,884,863 )

U.S. Treasury bills

          827,006,587             827,006,587  
                                 

December 31, 2014

                               

Futures contracts(a)

  $ (69,368,696 )   $     $     $ (69,368,696 )

U.S. Treasury bills

          784,772,217             784,772,217