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DERIVATIVES AND HEDGING ACTIVITIES - Summary of Fair Value, Notional Amount and Balance Sheet Classification (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Sep. 30, 2024
--- | --- | ---
Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional | $ 2,381,358 | $ 1,981,554
Derivative assets, fair value | 151,271 | 165,624
Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 2,687,017 | 3,614,512
Derivative liabilities, fair value | 73,330 | 48,055
Client swap program hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 977,017 | 1,044,512
Derivative assets, fair value | 37,347 | 46,758
Client swap program hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 977,017 | 1,044,512
Derivative liabilities, fair value | 37,818 | 47,388
Commercial loan fair value hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 34,341 | 37,042
Derivative assets, fair value | 1,611 | 1,595
Commercial loan fair value hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 0 | 0
Derivative liabilities, fair value | 0 | 0
Mortgage loan fair value hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 470,000 | 0
Derivative assets, fair value | 13,082 | 0
Mortgage loan fair value hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 1,100,000 | 2,570,000
Derivative liabilities, fair value | 20,426 | 667
Mortgage backed securities fair value hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 0 |
Derivative assets, fair value | 0 |
Mortgage backed securities fair value hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 610,000 |
Derivative liabilities, fair value | 15,086 |
Borrowings cash flow hedges | Other assets |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 900,000 | 900,000
Derivative assets, fair value | 99,231 | 117,271
Borrowings cash flow hedges | Other liabilities |  |
Derivatives, Fair Value [Line Items] |  |
Notional | 0 | 0
Derivative liabilities, fair value | $ 0 | $ 0

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