Commodity and Other Repurchase Agreements and Collateralized Transactions Gross Collateralized Financings by Collateral Type 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
US Government Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	3.0	7.0
US Government Agencies Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	342.6	332.6
Asset-backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	77.6	36.4
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	1,175.8	1,017.1
Equity Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned	$ 204.3	$ 111.1