Commodity and Other Repurchase Agreements and Collateralized Transactions Gross Financings Collateral Maturities Table (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 1,599.0	$ 1,393.1
Securities Loaned	204.3	111.1
Collateralized transactions:	1,803.3	1,504.2
Maturity Overnight and on Demand [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	699.1	640.2
Securities Loaned	204.3	111.1
Collateralized transactions:	903.4	751.3
Maturity Less than 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	584.9	432.9
Securities Loaned	0.0	0.0
Collateralized transactions:	584.9	432.9
Maturity 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	305.0	320.0
Securities Loaned	0.0	0.0
Collateralized transactions:	305.0	320.0
Maturity Greater than 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	10.0	0.0
Securities Loaned	0.0	0.0
Collateralized transactions:	$ 10.0	$ 0.0