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Significant Accounting Policies - Creations and Redemptions of Shares (Detail) - USD ($)
shares in Thousands, $ in Thousands
3 Months Ended 6 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2020
Mar. 31, 2019
Activity in Number of Shares Created and Redeemed:        
Creations     66,800 50,500
Redemptions     (50,400) (35,700)
Net Change in Number of Shares Created and Redeemed     16,400 14,800
Activity in Value of Shares Created and Redeemed:        
Creations $ 8,555,872 $ 2,820,652 $ 9,938,241 $ 6,027,193
Redemptions $ (4,620,586) $ (2,959,784) (7,254,854) (4,324,677)
Net Change in Value of Shares Created and Redeemed     $ 2,683,387 $ 1,702,516