Securities and Commodity Financing Transactions - Schedule of Collateral Maturities of Gross Obligations (Details) - USD ($) $ in Millions	Jun. 30, 2026	Sep. 30, 2025
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 36,381.9	$ 29,523.7
Securities loaned	2,955.7	2,550.8
Gross amount of secured financing	39,337.6	32,074.5
Overnight and Open
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	33,676.3	26,173.4
Securities loaned	2,955.7	2,550.8
Gross amount of secured financing	36,632.0	28,724.2
Less than 30 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	2,387.5	3,128.5
Securities loaned	0.0	0.0
Gross amount of secured financing	2,387.5	3,128.5
30-90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	272.7	180.4
Securities loaned	0.0	0.0
Gross amount of secured financing	272.7	180.4
Over 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	45.4	41.4
Securities loaned	0.0	0.0
Gross amount of secured financing	$ 45.4	$ 41.4

Amount, after the effects of master netting arrangements, of securities loaned to entities in exchange for collateral. Includes liabilities not subject to a master netting arrangement and not elected to be offset.