Derivatives and Hedging Activities - Impact of Fair Value Hedge Accounting on the Carrying Value of the Hedged Items (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 1,816,870	$ 1,159,496
Hedged Liability, Statement of Financial Position [Extensible Enumeration]	Loans receivable, net of allowance for loan losses of $177,207 and $172,808	Loans receivable, net of allowance for loan losses of $177,207 and $172,808
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	$ (48,865)	$ (39,090)
Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	1,780,503
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(45,622)	(36,458)
Hedged liability, amortized cost		1,119,975
Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	36,367	39,521
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(3,243)	(2,632)
Other liabilities
Derivatives, Fair Value [Line Items]
Notional	806,744	588,676
Other liabilities \| Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Notional	0	0
Other liabilities \| Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Notional	$ 0	$ 0

Nominal or face amount used to calculate payment on derivative.

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

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-1A

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+ References

No definition available.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 181
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480401/815-10-55-181

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
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-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-4EE

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- Definition

Amount of liability hedged in fair value hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-4EE

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4EE
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-4EE

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- Definition

Hedged Liability, Amortized Cost

+ References

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