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Derivatives and Hedging Activities - Impact of Fair Value Hedge Accounting on the Carrying Value of the Hedged Items (Details) - USD ($)	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 7,287,540,000	$ 1,816,870,000
Hedged Liability, Statement of Financial Position [Extensible Enumeration]	Loans receivable, net of allowance for loan losses of $203,753 and $177,207	Loans receivable, net of allowance for loan losses of $203,753 and $177,207
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	$ 20,005,000	$ (48,865,000)
Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Portfolio layer method, amortized cost	7,252,017,000	1,780,503,000
Portfolio layer method, cumulative adjustment, increase (decrease)	21,476,000	(45,622,000)
Amortized cost, designated in portfolio hedging relationship	2,570,000,000	670,000,000
Hedged asset, discontinued	300,000,000
Hedged asset, discontinued fair value	1,232,211
Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	35,523,000	36,367,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	$ (1,471,000)	$ (3,243,000)

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Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk, remaining after discontinued hedge.

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Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk.

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Indicates line item in statement of financial position that includes hedged liability.

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Amortized cost of closed portfolio of financial asset used to designate hedging relationship in which hedged item is expected to be outstanding for designated hedged period.

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