Securities and Commodity Financing Transactions - Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 9,196.4	$ 6,133.5
Securities loaned	1,117.3	1,189.5
Gross amount of secured financing	10,313.7	7,323.0
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	3,696.1	1,311.0
U.S. government agency obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	542.2	604.1
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	102.9	178.0
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	4,371.6	3,762.5
Foreign government obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	148.1	97.2
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	335.5	180.7
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 1,117.3	$ 1,189.5