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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024
Goodwill |  |
Goodwill, beginning of period | $ 411,360 | $ 304,750
Additions | 3,362 | 106,610
Goodwill, end of period | 414,722 | 411,360
Core Deposit and Other Intangibles |  |
Intangible assets, beginning of period | 37,065 | 5,869
Additions | 180 | 38,939
Amortization | (9,874) | (7,743)
Intangible assets, end of period | 27,371 | 37,065
Total |  |
Goodwill and intangible assets, beginning of period | 448,425 | 310,619
Additions | 3,542 | 145,549
Amortization | (9,874) | (7,743)
Goodwill and intangible assets, end of period | $ 442,093 | $ 448,425

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