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Business Combinations - Narrative (Details) $ in Millions |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2023 shares | Mar. 20, 2023 USD ($) branch | Mar. 19, 2023 USD ($) | Mar. 31, 2024 USD ($) | Sep. 30, 2024 USD ($) branch | Sep. 30, 2023 USD ($) | Dec. 31, 2023 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of traditional branches |  |  |  |  | 419 |  |
Settlement period |  |  |  |  | 1 year |  |
Bargain purchase gain | $ |  |  |  |  |  |  | $ 2,000
Reduction to bargain purchase gain | $ |  |  |  | $ 121 | $ 121 | $ (2,142) |
Signature Bridge Bank |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Bargain purchase gain | $ |  | $ 2,010 | $ 2,010 |  |  |  |
Signature Bridge Bank | FDIC |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Shares issued (in shares) | shares | 13,010,668 |  |  |  |  |  |
New York | Signature Bridge Bank |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of traditional branches |  | 29 |  |  |  |  |
California | Signature Bridge Bank |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of traditional branches |  | 7 |  |  |  |  |
North Carolina | Signature Bridge Bank |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of traditional branches |  | 2 |  |  |  |  |
Connecticut | Signature Bridge Bank |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of traditional branches |  | 1 |  |  |  |  |
Nevada | Signature Bridge Bank |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of traditional branches |  | 1 |  |  |  |  |

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