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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%

X

- Definition

The amount of expense during the period for floor brokerage fees paid to other broker-dealers to execute trades on their behalf, stock exchange fees, order flow fees, and clearance fees.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Percentage of Average Daily Net Assets

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