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Significant Accounting Policies - Creations and Redemptions of Shares (Detail) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Jun. 30, 2015
Sep. 30, 2014
Activity in Number of Shares Issued and Outstanding:    
Creations 44,500,000 47,200,000
Redemptions (63,300,000) (91,500,000)
Net increase/(decrease) Shares Issued and Outstanding (18,800,000) (44,300,000)
Activity in Value of Shares Issued and Outstanding:    
Creations $ 5,289,912 [1] $ 5,893,107
Redemptions (7,283,671) [1] (11,209,535)
Net increase/(decrease) Shares Issued and Outstanding $ (1,993,759) $ (5,316,428)
[1] Effective October 1, 2014, the Trust adopted the financial presentation provisions for an investment company. See note 2.2 to our unaudited financial statements.