Securities and Commodity Financing Transactions - Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 12,075.0	$ 9,196.4
Securities loaned	1,458.6	1,117.3
Gross amount of secured financing	13,533.6	10,313.7
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	9,017.4	3,696.1
U.S. government agency obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	99.1	542.2
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	40.9	102.9
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	2,302.5	4,371.6
Foreign government obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	318.2	148.1
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	296.9	335.5
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 1,458.6	$ 1,117.3