Commodity and Other Repurchase Agreements and Collateralized Transactions Gross Collateralized Financings by Collateral Type Table (Details) - USD ($)	Dec. 31, 2019	Sep. 30, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 2,931,600,000	$ 2,773,700,000
Securities Loaned	1,430,800,000	1,459,900,000
Collateralized transactions:	4,362,400,000	4,233,600,000
US Government Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	35,300,000	108,800,000
US Government Agencies Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	263,000,000	359,500,000
Asset-backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	84,500,000	96,700,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	2,548,800,000.0	2,208,700,000.0
Equity Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned	$ 1,430,800,000	$ 1,459,900,000