Physical Commodities Inventory - Physical Commodities Inventory by CIP and Finished (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Inventory [Line Items]
Physical Ag & Energy(1)	$ 160.2	$ 65.1
Precious metals - held by non-broker-dealer subsidiaries(3)	187.1	73.2
Physical commodities inventory	286.3	124.8
Physical commodities inventory - precious metals [Member]
Inventory [Line Items]
Precious metals - held by non-broker-dealer subsidiaries(3)	26.9	13.3
Finished commodities	$ 99.2	$ 46.4

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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