Physical Commodities Inventory - Physical Commodities Inventory by CIP and Finished (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016
Inventory [Line Items]
Physical Ag & Energy(1)	$ 139.5		$ 65.9
Precious metals - held by non-broker-dealer subsidiaries(3)		$ 117.8	71.2
Physical commodities inventory	181.7		123.8
Physical commodities inventory - precious metals [Member]
Inventory [Line Items]
Precious metals - held by non-broker-dealer subsidiaries(3)	17.0		5.3
Finished commodities	$ 25.2		$ 52.6