Securities and Commodity Financing Transactions - Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Jun. 30, 2023	Sep. 30, 2022
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 11,813.4	$ 6,133.5
Securities loaned	1,093.3	1,189.5
Gross amount of secured financing	12,906.7	7,323.0
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	6,169.3	1,311.0
U.S. government agency obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	676.5	604.1
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	122.6	178.0
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	4,425.9	3,762.5
Foreign government obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	130.6	97.2
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	288.5	180.7
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 1,093.3	$ 1,189.5