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Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - Income Statement Effects of Derivative Instruments (Details) - USD ($) $ in Millions | 3 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020
Interest Income |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Amount of gain reclassified from accumulated other comprehensive income into income | $ 0.1 |
Designated as Hedging Instrument | Interest Income |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Total derivatives designated as hedging instruments | 0.1 |
Amount of gain reclassified from accumulated other comprehensive income into income as a result of a forecasted transaction is no longer probable of occurring | 0.0 |
Interest rate contracts |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Total derivatives designated as hedging instruments | 25.4 | $ 12.6
Interest rate contracts | Interest Income |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Amount of gain reclassified from accumulated other comprehensive income into income | 0.1 |
Interest rate contracts | Designated as Hedging Instrument | Interest Income |  |
Derivative Instruments and Hedging Activities Disclosure [Line Items] |  |
Total derivatives designated as hedging instruments | $ 0.1 |

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- Definition Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Before Tax, Forecasted Transaction Not Probable + References No definition available. + Details
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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5618551-113959 + Details
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- Definition Amount, before tax, of reclassification of gain (loss) from accumulated other comprehensive income (AOCI) for derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e689-108580 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=124509347&loc=d3e637-108580 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5624171-113959 + Details
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