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Physical Commodities Inventory - Physical Commodities Inventory by CIP and Finished (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Sep. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016
--- | --- | --- | --- | ---
Inventory [Line Items] |  |  |  |
Physical Ag & Energy(1) | $ 125.1 | $ 65.1 |  |
Precious metals - held by non-broker-dealer subsidiaries(3) |  |  | $ 142.7 | $ 73.2
Physical commodities inventory | 244.7 | 124.8 |  |
Physical commodities inventory - precious metals [Member] |  |  |  |
Inventory [Line Items] |  |  |  |
Precious metals - held by non-broker-dealer subsidiaries(3) | 20.1 | 13.3 |  |
Finished commodities | $ 99.5 | $ 46.4 |  |

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