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Derivatives and Hedging Activities - Impact of Client Swap Program that are Not Designated in Accounting Hedges (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Jun. 30, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023
Client swap program hedges |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative instruments not designated as hedging instruments, gain (loss), net | $ 55 | $ (926) | $ 169 | $ (900)
Other noninterest income | Pay fixed/receive floating swap |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative instruments not designated as hedging instruments, gain (loss), net | (715) | 12,612 | (18,410) | (2,409)
Other noninterest income | Receive fixed/pay floating swap |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative instruments not designated as hedging instruments, gain (loss), net | $ 770 | $ (13,538) | $ 18,579 | $ 1,509

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