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Derivatives and Hedging Activities - Summary of Fair Value, Notional Amount and Balance Sheet Classification (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Sep. 30, 2023
--- | --- | ---
Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | $ 2,903,527 | $ 2,516,405
Derivative assets, fair value | 223,782 | 312,971
Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 896,485 | 806,744
Derivative liabilities, fair value | 56,392 | 79,668
Client swap program hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 896,485 | 806,744
Derivative assets, fair value | 55,631 | 78,797
Client swap program hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 896,485 | 806,744
Derivative liabilities, fair value | 56,392 | 79,668
Commercial loan fair value hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 37,042 | 39,661
Derivative assets, fair value | 2,061 | 3,405
Commercial loan fair value hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 0 | 0
Derivative liabilities, fair value | 0 | 0
Mortgage loan fair value hedges |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 970,000 | 670,000
Mortgage loan fair value hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 970,000 |
Derivative assets, fair value | 22,244 | 46,396
Mortgage loan fair value hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 0 | 0
Derivative liabilities, fair value | 0 | 0
Borrowings cash flow hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 1,000,000 | 1,000,000
Derivative assets, fair value | 143,846 | 184,373
Borrowings cash flow hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional amount | 0 | 0
Derivative liabilities, fair value | $ 0 | $ 0

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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/exampleRef -Topic 210 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 22 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483444/210-20-55-22 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 210 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483466/210-20-50-3 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 210 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483444/210-20-55-7 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480434/815-10-50-4B Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 4D -Subparagraph (c)(2) -SubTopic 20 -Topic 860 -Publisher FASB -URI https://asc.fasb.org//1943274/2147481326/860-20-50-4D + Details
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