Securities and Commodity Financing Transactions - Schedule of Underlying Collateral Types of Gross Obligations (Details) - USD ($) $ in Millions	Jun. 30, 2025	Sep. 30, 2024
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	$ 24,590.6	$ 16,527.9
Securities loaned	2,032.1	1,615.9
Gross amount of secured financing	26,622.7	18,143.8
U.S. Treasury obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	13,609.9	9,673.7
U.S. government agency obligations and municipal bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	354.3	652.0
Asset-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	172.3	136.1
Agency mortgage-backed obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	7,944.7	5,079.6
Foreign government obligations
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	939.5	649.6
Corporate bonds
Assets Sold under Agreements to Repurchase [Line Items]
Securities sold under agreements to repurchase	1,569.9	336.9
Equity securities
Assets Sold under Agreements to Repurchase [Line Items]
Securities loaned	$ 2,032.1	$ 1,615.9

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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