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Contracts and Agreements (Brokerage Commissions) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Long Duration Contracts Assumptions By Product And Guarantee [Line Items] |  |  |
Brokerage commissions | $ 19,025 | $ 4,643 | $ 9,848
Brokerage commissions as a percentage of average daily net assets | 0.03% | 0.01% | 0.01%
Rebalancing Investments Transaction [Member] |  |  |
Long Duration Contracts Assumptions By Product And Guarantee [Line Items] |  |  |
Brokerage commissions | $ 11,997 | $ 3,665 | $ 4,473
Brokerage commissions as a percentage of average daily net assets | 63.06% | 78.94% | 45.42%
Trades Necessitated By Creation And Redemption Activity [Member] |  |  |
Long Duration Contracts Assumptions By Product And Guarantee [Line Items] |  |  |
Brokerage commissions | $ 7,028 | $ 978 | $ 5,375
Brokerage commissions as a percentage of average daily net assets | 36.94% | 21.06% | 54.58%

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