Physical Commodities Inventory - Physical Commodities Inventory by CIP and Finished (Details) - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016
Inventory [Line Items]
Physical Ag & Energy(1)	$ 137.5	$ 65.9
Precious metals - held by non-broker-dealer subsidiaries(3)	129.8	71.2
Physical commodities inventory	212.1	123.8
Physical commodities inventory - precious metals [Member]
Inventory [Line Items]
Precious metals - held by non-broker-dealer subsidiaries(3)	18.1	5.3
Finished commodities	$ 56.5	$ 52.6