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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, 2024 | Sep. 30, 2023
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Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | $ 8,040,874 | $ 1,816,870
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items | (39,639) | (48,865)
Mortgage loan fair value hedges |  |
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | 8,006,330 | 1,780,503
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items | (37,192) | (45,622)
Notional amount | 3,070,000 | 670,000
Commercial loan fair value hedges |  |
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | 34,544 | 36,367
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items | $ (2,447) | $ (3,243)

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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/exampleRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 181 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480401/815-10-55-181 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4EE -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480434/815-10-50-4EE + 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 -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4EE -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480434/815-10-50-4EE + Details
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