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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
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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

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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 + Details
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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 + Details
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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 + Details
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