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DERIVATIVES AND HEDGING ACTIVITIES - Gains/(Losses) on Derivative Instruments in Fair Value and Cash Flow Accounting Hedging Relationships (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Interest income/(expense), including the effects of fair value and cash flow hedges | $ 1,119,937 | $ 1,165,849 | $ 900,068
Interest on mortgage-backed securities | 103,071 | 59,782 | 43,184
Interest income/(expense), including the effects of fair value and cash flow hedges | (80,507) | (178,444) | $ (115,488)
Interest rate contracts | Interest income on loans receivable | Fair value hedging |  |  |
Gain/(loss) on fair value hedging relationships: |  |  |
Amounts related to interest settlements on derivatives | 20,641 | 39,223 |
Recognized on derivatives | 2,222 | (67,785) |
Recognized on hedged items | (9,194) | 67,639 |
Net income/(expense) recognized on fair value hedges | 13,669 | 39,077 |
Interest rate contracts | Interest on mortgage-backed securities | Fair value hedging |  |  |
Gain/(loss) on fair value hedging relationships: |  |  |
Amounts related to interest settlements on derivatives | 962 | 0 |
Recognized on derivatives | (15,086) | 0 |
Recognized on hedged items | 15,452 | 0 |
Net income/(expense) recognized on fair value hedges | 1,328 | 0 |
Interest rate contracts | Interest expense on FHLB advances | Cash flow hedging |  |  |
Gain/(loss) on cash flow hedging relationships: |  |  |
Amounts related to interest settlements on derivatives | 35,474 | 46,645 |
Amount of derivative gain/(loss) reclassified from AOCI into interest income/expense | 0 | 0 |
Net income/(expense) recognized on cash flow hedges | $ 35,474 | $ 46,645 |

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- Definition Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 21 -Subparagraph (i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147476148/220-40-50-21 + Details
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- Definition Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4C -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4C Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 182 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480401/815-10-55-182 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4A -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 21 -Subparagraph (i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147476148/220-40-50-21 + Details
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