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Derivatives and Hedging Activities - Impact of Fair Value Hedge Accounting on the Carrying Value of the Hedged Items (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | $ 7,287,540,000 | $ 1,816,870,000
Hedged Liability, Statement of Financial Position [Extensible Enumeration] | Loans receivable, net of allowance for loan losses of $203,753 and $177,207 | Loans receivable, net of allowance for loan losses of $203,753 and $177,207
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items | $ 20,005,000 | $ (48,865,000)
Mortgage loan fair value hedges |  |
Derivatives, Fair Value [Line Items] |  |
Portfolio layer method, amortized cost | 7,252,017,000 | 1,780,503,000
Portfolio layer method, cumulative adjustment, increase (decrease) | 21,476,000 | (45,622,000)
Amortized cost, designated in portfolio hedging relationship | 2,570,000,000 | 670,000,000
Hedged asset, discontinued | 300,000,000 |
Hedged asset, discontinued fair value | 1,232,211 |
Commercial loan fair value hedges |  |
Derivatives, Fair Value [Line Items] |  |
Carrying value of hedged items | 35,523,000 | 36,367,000
Cumulative gain (loss) fair value hedge adjustment included in carrying amount of hedged items | $ (1,471,000) | $ (3,243,000)

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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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