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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 | 6 Months Ended | 12 Months Ended
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Mar. 31, 2023 | Sep. 30, 2022
Derivatives, Fair Value [Line Items] |  |
Asset Derivatives | $ 9,275.0 | $ 9,657.2
Liability Derivatives | 9,133.7 | 9,552.4
Designated as Hedging Instrument |  |
Derivatives, Fair Value [Line Items] |  |
Asset Derivatives | 4.7 | 0.0
Derivative assets expected to be released from Other comprehensive income into current earnings: | 0.5 | 0.0
Liability Derivatives | 38.5 | 70.6
Derivative liabilities expected to be released from Other comprehensive income into current earnings: | 32.7 | 18.6
Designated as Hedging Instrument | Interest rate contracts |  |
Derivatives, Fair Value [Line Items] |  |
Asset Derivatives | 1.8 | 0.0
Liability Derivatives | 38.4 | 48.8
Derivative liabilities expected to be released from Other comprehensive income into current earnings: | 32.7 | 9.7
Designated as Hedging Instrument | Foreign currency forward contracts |  |
Derivatives, Fair Value [Line Items] |  |
Asset Derivatives | 2.9 | 0.0
Derivative assets expected to be released from Other comprehensive income into current earnings: | 0.5 | 0.0
Liability Derivatives | 0.1 | 21.8
Derivative liabilities expected to be released from Other comprehensive income into current earnings: | $ 0.0 | $ 8.9

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- Definition Fair value, before effects of master netting arrangements, of a financial asset or other 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 assets elected not to be offset. Excludes assets not subject to a master netting arrangement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 22 -URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4D -Subparagraph (c)(2) -URI https://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5624163-113959 + Details
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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4D -Subparagraph (c)(2) -URI https://asc.fasb.org/extlink&oid=121570589&loc=SL51823488-111719 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313 Reference 5: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 22 -URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313 + Details
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