Securities and Commodity Financing Transactions - Carrying Value of Collateral Pledged, Received and Repledged (Details) - USD ($) $ in Millions	Mar. 31, 2024	Sep. 30, 2023
Assets Sold under Agreements to Repurchase [Line Items]
Securities pledged or repledged to cover collateral requirements for tri-party arrangements	$ 5,666.5	$ 5,044.8
Securities received as collateral that may be repledged	14,131.4	9,180.1
Securities received as collateral that may be repledged covering securities sold short	2,563.7	2,461.1
Asset Pledged as Collateral with Right
Assets Sold under Agreements to Repurchase [Line Items]
Repledged securities borrowed and client securities held under custodial clearing arrangements to collateralize securities loaned agreements	1,298.5	1,097.3
Tri-party Repurchase Agreements
Assets Sold under Agreements to Repurchase [Line Items]
Securities pledged or repledged to cover collateral requirements for tri-party arrangements	$ 5,827.0	$ 4,726.6

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(m)(1)(ii)(A))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

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Fair value of collateral permitted, by contact or custom, to be sold or re-pledged.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481420/860-30-50-1A

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Fair value of the collateral, or portion thereof, that has been re-pledged, delivered, or otherwise used.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481420/860-30-50-1A

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The aggregate fair value as of the balance sheet date of financial instruments and other positions owned by the entity including: (1) mortgages, mortgage-backed and asset backed securities; (2) US government and agency obligations; (3) state and municipal government obligations; (4) other sovereign government debt; (5) corporate obligations; (6) corporate equities; (7) principal investments; (8) derivative contracts; and (9) physical commodities. Includes both pledged and unpledged holdings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-Publisher FASB
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Fair value of the securities pledged as collateral against securities loaned.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483444/210-20-55-12

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