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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	Mar. 31, 2025
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 7,287,540,000	$ 6,265,535,000
Hedged Liability, Statement of Financial Position [Extensible Enumeration]	Net loans
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	$ 20,005,000	1,087,000
Loans receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items		5,674,948,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items		(7,814,000)
Available-for-sale securities, at fair value
Derivatives, Fair Value [Line Items]
Carrying value of hedged items		590,587,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items		8,901,000
Other assets
Derivatives, Fair Value [Line Items]
Notional amount	1,981,554,000	3,532,069,000
Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Portfolio layer method, amortized cost	7,252,017,000	5,642,530,000
Portfolio layer method, cumulative adjustment, increase (decrease)	21,476,000	(5,937,000)
Amortized cost, designated in portfolio hedging relationship		1,570,000,000
Hedged asset, discontinued	300,000,000
Hedged asset, discontinued fair value	1,232,211
Mortgage loan fair value hedges | Other assets
Derivatives, Fair Value [Line Items]
Notional amount	0	1,570,000,000
Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	35,523,000	32,418,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(1,471,000)	(1,877,000)
Commercial loan fair value hedges | Other assets
Derivatives, Fair Value [Line Items]
Notional amount	$ 37,042,000	34,341,000
Mortgage backed securities fair value hedges
Derivatives, Fair Value [Line Items]
Carrying value of hedged items		590,587,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items		8,901,000
Amortized cost, designated in portfolio hedging relationship		400,000,000
Mortgage backed securities fair value hedges | Other assets
Derivatives, Fair Value [Line Items]
Notional amount		$ 0

X

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1B

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+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
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-Section 55
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Reference 1: http://www.xbrl.org/2003/role/exampleRef
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Reference 1: http://www.xbrl.org/2003/role/exampleRef
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