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Fair Value Measurements (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Reconciliation of changes in fair value measurement, assets [abstract]    
Balance at beginning of year $ 1,026,685  
Realized gains (losses) on sales and redemptions for physical bullion 759 $ (16,398)
Change in unrealized gains (losses) (1,026,156) (455,536)
Balance at end of year 1,594,588 1,026,685
Physical gold bullion cost 2,100,112 1,706,026
Physical silver bullion cost 1,036,069 993,993
Recurring fair value measurement    
Reconciliation of changes in fair value measurement, assets [abstract]    
Balance at beginning of year 2,959,687 2,808,052
Purchases 440,262  
Sales (5,043) (23,265)
Redemptions for physical bullion 184 (264,238)
Realized gains (losses) on sales and redemptions for physical bullion 759 (16,398)
Change in unrealized gains (losses) 1,026,156 455,536
Balance at end of year $ 4,422,005 $ 2,959,687