Derivatives and Hedging Activities - Impact of Fair Value Hedge Accounting on the Carrying Value of the Hedged Items (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 2,855,203	$ 1,612,208
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	26,830	2,680
Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	17,605	(1,608)
Amortized cost basis of the hedged loans		200,000
Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	9,225	4,288
Loans Receivable
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	2,855,203	1,612,208
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items	26,830	2,680
Loans Receivable \| Mortgage loan fair value hedges
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	2,752,843	1,520,647
Loans Receivable \| Commercial loan fair value hedges
Derivatives, Fair Value [Line Items]
Carrying value of hedged items	$ 102,360	$ 91,561

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.

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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