Derivatives and Hedging Activities - Impact of Fair Value Hedge Accounting on the Carrying Value of the Hedged Items (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Sep. 30, 2022
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 1,917,364	$ 1,159,496
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(29,262)	(39,090)
Loans Receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	1,917,364	1,159,496
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(29,262)	(39,090)
Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	700,058	588,676
Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(27,338)	(36,458)
Notional amount	670,000
Mortgage loan fair value hedges | Loans Receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	1,879,678	1,119,975
Mortgage loan fair value hedges | Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	0	0
Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(1,924)	(2,632)
Commercial loan fair value hedges | Loans Receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	37,686	40,000
Commercial loan fair value hedges | Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	$ 0	$ 0