ALLOWANCE FOR LOAN LOSSES - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Financing Receivable, Allowance for Credit Loss [Line Items]
Provision for credit losses	$ 7,750	$ 17,500	$ 41,500
Allowance for credit loss, charge-off (recovery)	11,783	1,356
Gross loans	20,288,338	21,120,107
Allowance for loan losses	199,720	203,753	$ 177,207
Loans Subject to Quantitative Evaluation
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	131,652
Loans Subject to Management Qualitative Evaluation
Financing Receivable, Allowance for Credit Loss [Line Items]
Allowance for loan losses	68,068
Non-Performing Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Gross loans	$ 143,022	$ 77,418
Ratio of non-performing assets to total assets	0.54%	0.28%
Financial Asset Originated
Financing Receivable, Allowance for Credit Loss [Line Items]
Non-accrual loans	$ 128,628	$ 69,541

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

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Amount, excluding accrued interest, of decrease (increase) in allowance for credit loss on financing receivable from writeoff (recovery). Excludes net investment in lease.

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Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

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Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

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Amortized cost excluding accrued interest, of financing receivable on nonaccrual status. Excludes net investment in lease.

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