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Commitments and contingencies - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Dec. 31, 2024
Loss Contingencies [Line Items] |  |  |
Commitments to extend credit to commercial customers | $ 12,800,000 |  | $ 12,700,000
Maximum credit risk for recourse associated with loans sold under Federal National Mortgage Association Delegated Underwriting and Servicing program | 4,400,000 |  | 4,200,000
Loss contingency accrual | 75,000 |  | $ 157,000
Loss contingency accrual, period decrease | 8,000 |  |
Estimate assessed losses |  | $ 34,000 |
Minimum |  |  |
Loss Contingencies [Line Items] |  |  |
Range of reasonably possible losses | 0 |  |
Maximum |  |  |
Loss Contingencies [Line Items] |  |  |
Range of reasonably possible losses | $ 25,000 |  |

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