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Allowance for Loan Losses - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Sep. 30, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Recoveries, net of charge-offs | $ 113 | $ 489 |
Gross loans | $ 17,763,942 |  | $ 17,653,757
Percent past due | 0.33% |  | 0.36%
Financial Asset Originated |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Non-accrual loans | $ 45,258 |  | $ 50,422
Non-Performing Loans |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Gross loans | $ 55,388 |  | $ 57,924
Ratio of non-performing assets to total assets | 0.24% |  | 0.26%
Commercial loans |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Gross loans | $ 10,563,949 |  | $ 10,507,610
Percent past due | 0.31% |  | 0.31%
Commercial loans | Financial Asset Originated |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Non-accrual loans | $ 28,852 |  | $ 34,644
Commercial loans | Commercial & industrial |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Gross loans | $ 2,364,982 |  | $ 2,315,318
Percent past due | 0.30% |  | 0.36%
Commercial loans | Commercial & industrial | Financial Asset Originated |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Non-accrual loans | $ 4,437 |  | $ 6,082

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