Securities and Commodity Financing Transactions - Schedule of Underlying Collateral Types 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
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	20,383.4	19,311.9
U.S. government agency obligations and municipal bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	1,773.3	804.3
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	477.6	55.0
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	10,532.6	7,521.4
Foreign government obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	1,194.5	881.4
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	2,020.5	949.7
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 2,955.7	$ 2,550.8

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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