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Derivative Contract Information - Summary of Average Daily Notional Value of Derivative Contracts Outstanding (Detail) - Over-the-Counter ("OTC") [Member] - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Gold Delivery Agreement Average notional $ 17,674,000 $ 22,963,000
SPDR Long Dollar Gold Trust [Member]    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Gold Delivery Agreement Average notional $ 17,674,000 $ 22,963,000