Securities and Commodity Financing Transactions - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2022	Sep. 30, 2021
Assets Sold under Agreements to Repurchase [Line Items]
Fair value of financial instruments owned	$ 4,167.3	$ 4,354.6
Fair value of repledged securities	2,372.3	843.3
Securities loaned, fair value of collateral	1,146.0	2,097.6
Fair value of securities received as collateral that can be resold or repledged	5,836.1	4,399.8
Fair value of securities received as collateral that have been resold or repledged	1,615.3	1,031.1
Asset Pledged as Collateral with Right
Assets Sold under Agreements to Repurchase [Line Items]
Fair value of financial instruments owned	2,372.3	843.3
Asset Pledged as Collateral without Right \| Tri-party Repurchase Agreements
Assets Sold under Agreements to Repurchase [Line Items]
Fair value of financial instruments owned	822.5	2,359.6
Fair value of repledged securities	$ 2,965.3	$ 1,157.9
Repurchase agreement term	2 years

Securities Sold under Agreements to Repurchase, Term

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+ References

No definition available.

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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
-URI https://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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- Definition

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-URI https://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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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.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-URI https://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 12
-URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226013-175313

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Fair value of securities pledged as collateral against securities sold under agreement to repurchase.

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