Commodity and Other Repurchase Agreements and Collateralized Transactions Gross Collateralized Financings by Collateral Type Table (Details) - USD ($)	Mar. 31, 2019	Sep. 30, 2018
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 2,384,700,000	$ 1,936,700,000
Securities Loaned	1,376,500,000	277,900,000
Collateralized transactions:	3,761,200,000	2,214,600,000
US Government Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	122,600,000	39,600,000
US Government Agencies Debt Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	372,800,000	461,700,000
Asset-backed Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	86,500,000	50,000,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	1,802,800,000.0	1,385,400,000.0
Equity Securities [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Securities Loaned	$ 1,376,500,000	$ 277,900,000