Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Fair Values of Derivative Instruments Designated for Hedging Held (Details) - USD ($) $ in Millions	Jun. 30, 2022	Sep. 30, 2021
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 12,309.7	$ 5,760.7
Liability Derivatives	12,254.7	5,628.1
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.0	0.0
Liability Derivatives	33.3	$ 0.0
Designated as Hedging Instrument \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.0
Designated as Hedging Instrument \| Interest rate contracts \| Financial instruments sold, not yet purchased
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 33.3

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
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-URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

Name:	us-gaap_DerivativeFairValueOfDerivativeLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant