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Derivatives and Hedging Activities - Impact of Client Swap Program that are Not Designated in Accounting Hedges (Details) - USD ($) | 12 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022
Interest rate contracts |  |
Derivative [Line Items] |  |
Derivative instruments not designated as hedging instruments, gain (loss), net | $ (870,000) | $ 0
Other noninterest income | Pay fixed/receive floating swap |  |
Derivative [Line Items] |  |
Derivative instruments not designated as hedging instruments, gain (loss), net | 11,544,000 | 78,244,000
Other noninterest income | Receive fixed/pay floating swap |  |
Derivative [Line Items] |  |
Derivative instruments not designated as hedging instruments, gain (loss), net | $ (12,414,000) | $ (78,244,000)

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