Derivatives and Hedging Activities - Schedule of Fair Value Hedge Accounting on Carrying Value of Hedged Items (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2026	Sep. 30, 2025
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 6,052,202,000	$ 6,366,196,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	9,769,000	22,246,000
Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Portfolio layer method, amortized cost	5,145,220,000	5,393,257,000
Portfolio layer method, cumulative adjustment, increase (decrease)	1,305,000	8,262,000
Amortized cost, designated in portfolio hedging relationship	1,370,000,000	1,570,000,000
Hedged asset, discontinued		1,600,000,000
Hedged asset, discontinued fair value		4,016,668
Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Carrying value of hedged items		32,829,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items		(1,468,000)
Hedged asset, discontinued	31,562,000
Hedged asset, discontinued fair value	(1,502,681)
Mortgage backed securities fair value hedges
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	906,982,000	940,110,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	8,464,000	15,452,000
Amortized cost, designated in portfolio hedging relationship	610,000,000	610,000,000
Loans receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 5,145,220,000	$ 5,426,086,000
Hedged Liability, Statement of Financial Position [Extensible Enumeration]	Net loans	Net loans
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	$ 1,305,000	$ 6,794,000
Available-for-sale securities, at fair value
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 906,982,000	$ 940,110,000
Hedged Liability, Statement of Financial Position [Extensible Enumeration]	Available-for-sale securities, at fair value	Available-for-sale securities, at fair value
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	$ 8,464,000	$ 15,452,000

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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. Excludes cumulative basis adjustment related to foreign exchange risk.

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