Derivatives and Hedging Activities - Impact of Fair Value Hedge Accounting on the Carrying Value of the Hedged Items (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 1,515,487	$ 2,562,765
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	4,215	24,664
Loans receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	1,515,487	2,562,765
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	4,215	24,664
Other liabilities
Derivatives, Fair Value [Line Items]
Notional	1,159,033	2,849,390
Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	1,864	16,049
Hedged asset, discontinued fair value	$ 30,000
Hedged asset, discontinued fair value (percent)	30.00%
Hedged asset, total last of layer hedge	$ 100,000
Unamortized discount, termination portion of hedge	1,238
Mortgage loan fair value hedges \| Loans receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	1,468,517	2,461,008
Mortgage loan fair value hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional	470,000	500,000
Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	2,351	8,615
Hedged asset, discontinued fair value	46,240
Commercial loan fair value hedges \| Loans receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	46,970	101,757
Commercial loan fair value hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional	$ 44,678	$ 93,316

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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