Physical Commodities Inventory - Physical Commodities Inventory (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020
Inventory [Line Items]
Physical Ag & Energy	$ 261.8	$ 201.5
Precious metals - held by broker-dealer subsidiary	355.2	215.7
Physical commodities inventory, net	450.8	281.1
Physical Commodities Inventory - Precious Metals
Inventory [Line Items]
Precious metals - held by broker-dealer subsidiary	93.4	14.2
Precious metals - held by non-broker-dealer subsidiaries	$ 95.6	$ 65.4

Physical commodities inventory at fair value - precious metals

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Carrying amount as of the balance sheet date of merchandise or supplies held which are related to plants and animals.

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Carrying amount, net of valuation reserves and adjustments, as of the balance sheet date of merchandise or goods held by the company that are readily available for sale.

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Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

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