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Derivative Contract Information - Summary of Average Daily Notional Value of Derivative Contracts Outstanding (Detail) - USD ($)
3 Months Ended 9 Months Ended
Jun. 30, 2017
Jun. 30, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Gold Delivery Agreement Average notional $ 27,153,000 $ 25,851,000
SPDR Long Dollar Gold Trust [Member]    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Gold Delivery Agreement Average notional $ 27,153,000 $ 25,851,000