Commodity and Other Repurchase Agreements and Collateralized Transactions Gross Financings Collateral Maturities Table (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 1,562.5	$ 1,393.1
Securities Loaned	559.1	111.1
Collateralized transactions:	2,121.6	1,504.2
Maturity Overnight and on Demand [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	797.7	640.2
Securities Loaned	559.1	111.1
Collateralized transactions:	1,356.8	751.3
Maturity Less than 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	464.3	432.9
Securities Loaned	0.0	0.0
Collateralized transactions:	464.3	432.9
Maturity 30 to 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	280.5	320.0
Securities Loaned	0.0	0.0
Collateralized transactions:	280.5	320.0
Maturity Greater than 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	20.0	0.0
Securities Loaned	0.0	0.0
Collateralized transactions:	$ 20.0	$ 0.0