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Derivatives and Hedging Activities - Impact of Fair Value Hedge Accounting on the Carrying Value of the Hedged Items (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 1,934,688	$ 2,562,765
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	2,831	24,664
Loans Receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	1,934,688	2,562,765
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	2,831	24,664
Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	(1,929)	16,049
Amortized cost basis of the hedged loans	500,000
Mortgage loan fair value hedges | Loans Receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	1,839,169	2,461,008
Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	4,760	8,615
Commercial loan fair value hedges | Loans Receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 95,519	$ 101,757

X

- Definition

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.

+ 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109999712-113959

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- Definition

Amount of asset hedged in last of layer, expected to be remaining at end of 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 4EEE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109999718-113959

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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=121590274&loc=SL109999712-113959

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