Derivative Instruments - Summary of Average Monthly Notional Value of Future Contracts Outstanding (Detail) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted Average [Member] \| Currency Futures Contracts [Member]
Derivative [Line Items]
Average Notional Value	$ 659,935,078	$ 856,900,095	$ 1,169,610,341

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.

No definition available.

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