Derivatives and Hedging Activities - Impact of Client Swap Program that are Not Designated in Accounting Hedges (Details) - USD ($)	12 Months Ended
	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)