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Physical Commodities Inventory - Physical Commodities Inventory (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Sep. 30, 2020
--- | --- | ---
Inventory [Line Items] |  |
Physical Ag & Energy | $ 280.7 | $ 201.5
Precious metals - held by broker-dealer subsidiary | 299.5 | 215.7
Physical commodities inventory | 439.1 | 281.1
Physical Commodities Inventory - Precious Metals |  |
Inventory [Line Items] |  |
Precious metals - held by broker-dealer subsidiary | 18.8 | 14.2
Precious metals - held by non-broker-dealer subsidiaries | $ 139.6 | $ 65.4

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