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FAIR VALUE MEASUREMENTS - Fair Value by Balance Sheet Grouping (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Sep. 30, 2024
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | $ 3,533,201 | $ 2,572,709
Held-to-maturity securities | 645,802 | 436,972
FHLB stock | 88,068 | 95,617
Carrying Amount |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 3,533,201 | 2,572,709
Held-to-maturity securities | 645,802 | 436,972
Carrying Amount | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 657,310 | 2,381,102
Carrying Amount | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
FHLB stock | 88,068 | 95,617
Time deposits | 9,131,104 | 9,556,785
Borrowings | 1,765,604 | 3,267,589
Carrying Amount | Level 2 | Client swap program hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other assets | 37,347 | 46,758
Other liabilities | 37,818 | 47,388
Carrying Amount | Level 2 | Commercial loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other assets | 1,611 | 1,595
Carrying Amount | Level 2 | Mortgage loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other assets | 13,082 | 0
Other liabilities | 20,426 | 667
Carrying Amount | Level 2 | Mortgage backed securities fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other liabilities | 15,086 | 0
Carrying Amount | Level 2 | Borrowings cash flow hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other assets | 99,231 | 117,271
Carrying Amount | Level 2 | U.S. government and agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 235,919 | 314,204
Carrying Amount | Level 2 | Asset-backed securities due |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 506,334 | 540,125
Carrying Amount | Level 2 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 35,258 | 35,073
Carrying Amount | Level 2 | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 152,537 | 296,282
Carrying Amount | Level 2 | Agency pass-through certificates |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 2,603,153 | 1,387,025
Held-to-maturity securities | 645,802 | 436,972
Carrying Amount | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Loans receivable | 20,088,618 | 20,916,354
Junior subordinated deferrable interest debentures | 51,645 | 50,718
Estimated Fair Value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 3,533,201 | 2,572,709
Held-to-maturity securities | 612,739 | 401,046
Estimated Fair Value | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 657,310 | 2,381,102
Estimated Fair Value | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
FHLB stock | 88,068 | 95,617
Time deposits | 9,121,470 | 9,787,187
Borrowings | 1,755,130 | 3,276,122
Estimated Fair Value | Level 2 | Client swap program hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other assets | 37,347 | 46,758
Other liabilities | 37,818 | 47,388
Estimated Fair Value | Level 2 | Commercial loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other assets | 1,611 | 1,595
Estimated Fair Value | Level 2 | Mortgage loan fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other assets | 13,082 | 0
Other liabilities | 20,426 | 667
Estimated Fair Value | Level 2 | Mortgage backed securities fair value hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other liabilities | 15,086 | 0
Estimated Fair Value | Level 2 | Borrowings cash flow hedges |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Other assets | 99,231 | 117,271
Estimated Fair Value | Level 2 | U.S. government and agency securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 235,919 | 314,204
Estimated Fair Value | Level 2 | Asset-backed securities due |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 506,334 | 540,125
Estimated Fair Value | Level 2 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 35,258 | 35,073
Estimated Fair Value | Level 2 | Corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 152,537 | 296,282
Estimated Fair Value | Level 2 | Agency pass-through certificates |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale securities, at fair value | 2,603,153 | 1,387,025
Held-to-maturity securities | 612,739 | 401,046
Estimated Fair Value | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Loans receivable | 19,681,909 | 20,269,059
Junior subordinated deferrable interest debentures | $ 50,925 | $ 50,240

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