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Significant Accounting Policies - Additional Information (Detail)
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
Mar. 31, 2018
USD ($)
Mar. 31, 2019
USD ($)
t
shares
Mar. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
t
Accounting Policies [Abstract]          
Fair value of assets and liabilities, amount transferred between levels     $ 0   $ 0
Gold held by subcustodian | t     0   0
Maximum number of business days within which ownership of the gold is transferred     2 days    
Minimum denomination of shares issued redeemed against gold | shares     100,000    
Net realized and change in unrealized gain/(loss) on investment in gold $ 377,585,000 $ 742,014,000 $ 2,690,035,000 $ 1,096,754,000  
Net realized gain/(loss) from investment in gold sold to pay expenses 1,092,000 1,918,000 269,000 2,508,000  
Net realized gain/(loss) from gold distributed for the redemption of shares 120,555,000 148,686,000 82,967,000 187,077,000  
Net change in unrealized appreciation/(depreciation) on investment in gold 255,938,000 $ 591,410,000 2,606,799,000 $ 907,169,000  
Liability for uncertain tax positions - current $ 0   $ 0   $ 0