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Physical Commodities Inventory - Physical Commodities Inventory by CIP and Finished (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Inventory [Line Items]      
Physical Ag & Energy(1) $ 108.6   $ 65.9
Precious metals - held by non-broker-dealer subsidiaries(3)   $ 84.4 71.2
Physical commodities inventory 172.5   123.8
Physical commodities inventory - precious metals [Member]      
Inventory [Line Items]      
Precious metals - held by non-broker-dealer subsidiaries(3) 11.6   5.3
Finished commodities $ 52.3   $ 52.6