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Contracts and Agreements (Brokerage Commissions) (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]        
Brokerage commissions $ 5,205 $ 1,076 $ 15,370 $ 3,592
Brokerage commissions as a percentage of average daily net assets 0.03% 0.01% 0.03% 0.01%
Rebalancing Investments Transaction [Member]        
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]        
Brokerage commissions     $ 9,525 $ 2,908
Brokerage commissions as a percentage of average daily net assets 61.97% 80.96% 61.97% 80.96%
Trades Necessitated By Creation And Redemption Activity [Member]        
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]        
Brokerage commissions     $ 5,845 $ 684
Brokerage commissions as a percentage of average daily net assets 38.03% 19.04% 38.03% 19.04%