Physical Commodities Inventory - Physical Commodities Inventory by CIP and Finished (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Inventory [Line Items]
Physical Ag & Energy(1)	$ 151.1	$ 114.7
Precious metals - held by non-broker-dealer subsidiaries(3)	164.4	156.9
Physical commodities inventory	222.5	222.5
Physical commodities inventory - precious metals [Member]
Inventory [Line Items]
Precious metals - held by non-broker-dealer subsidiaries(3)	13.3	42.1
Finished commodities	$ 58.1	$ 65.7