Derivatives and Hedging Activities - Impact of Fair Value Hedge Accounting on the Carrying Value of the Hedged Items (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Sep. 30, 2021
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 1,272,018	$ 1,515,487
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(18,845)	4,215
Loans Receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	1,272,018	1,515,487
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(18,845)	4,215
Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	658,039	1,159,033
Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(19,000)	1,900
Notional amount	470,000
Hedged asset, discontinued		$ 30,000
Hedged asset, discontinued, percentage		30.00%
Hedged asset, discontinued, last layer hedge		$ 100,000
Mortgage loan fair value hedges \| Loans Receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	1,230,000	1,469,000
Mortgage loan fair value hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	0	470,000
Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	100	2,400
Hedged asset, discontinued, last layer hedge		46,000
Commercial loan fair value hedges \| Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	1,000
Commercial loan fair value hedges \| Loans Receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	42,000	47,000
Commercial loan fair value hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	$ 42,209	$ 44,678

Nominal or face amount used to calculate payment on derivative.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL109999712-113959

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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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL109999712-113959

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- Definition

Amount of liability hedged in fair value hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL109999712-113959

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