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Significant Accounting Policies - Creations and Redemptions of Shares (Detail) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Mar. 31, 2018
Sep. 30, 2017
Activity in Number of Shares Issued and Outstanding:    
Creations 32,200,000 92,500,000
Redemptions (37,900,000) (119,400,000)
Net increase/(decrease) in Number of Shares Issued and Outstanding (5,700,000) (26,900,000)
Activity in Value of Shares Issued and Outstanding:    
Creations $ 4,003,820 $ 11,161,181
Redemptions (4,686,586) (13,903,151)
Net increase/(decrease) in Value of Shares Issued and Outstanding $ (682,766) $ (2,741,970)