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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 | Sep. 30, 2020 | Sep. 30, 2019
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Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | $ 2,562,765 | $ 1,612,208
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items | 24,664 | 2,680
Loans receivable |  |
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | 2,562,765 | 1,612,208
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items | 24,664 | (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 | 16,049 | 1,608
Amortized cost basis of the hedged loans | 500,000 | 200,000
Mortgage loan fair value hedges | Loans receivable |  |
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | 2,461,008 | 1,520,647
Commercial loan fair value hedges |  |
Derivatives, Fair Value [Line Items] |  |
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items | 8,615 | (4,288)
Commercial loan fair value hedges | Loans receivable |  |
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | $ 101,757 | $ 91,561

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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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