Significant Accounting Policies - Creations and Redemptions of Shares (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Activity in Number of Shares Created and Redeemed:
Creations			50,500	32,200
Redemptions			(35,700)	(37,900)
Net Change in Number of Shares Created and Redeemed			14,800	(5,700)
Activity in Value of Shares Created and Redeemed:
Creations	$ 2,820,652	$ 2,908,789	$ 6,027,193	$ 4,003,820
Redemptions	$ (2,959,784)	$ (2,512,680)	(4,324,677)	(4,686,586)
Net Change in Value of Shares Created and Redeemed			$ 1,702,516	$ (682,766)

Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).

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-Name Accounting Standards Codification
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Period Type:	duration