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CONTRACTS AND AGREEMENTS (Brokerage Commissions) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]
Brokerage commissions	$ 17,897	$ 22,207	$ 19,025
Brokerage commissions as a percentage of average daily net assets	0.02%	0.02%	0.03%
Rebalancing Investments Transaction [Member]
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]
Brokerage commissions	$ 13,735	$ 17,872	$ 11,997
Brokerage commissions as a percentage of average daily net assets	76.74%	80.48%	63.06%
Trades Necessitated By Creation And Redemption Activity [Member]
Long Duration Contracts Assumptions By Product And Guarantee [Line Items]
Brokerage commissions	$ 4,162	$ 4,335	$ 7,028
Brokerage commissions as a percentage of average daily net assets	23.26%	19.52%	36.94%

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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