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Acquisition and Divestitures - Summary of Merger Related Expenses (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Outside data processing and software | $ 492 | $ 437 | $ 376
Advertising and marketing | $ 104 | $ 108 | 90
Peoples United Financial Inc |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Salaries and employee benefits |  |  | 102
Equipment and net occupancy |  |  | 7
Outside data processing and software |  |  | 5
Professional and other services |  |  | 72
Advertising and marketing |  |  | 9
Other cost of operations |  |  | 143
Other Expenses, Total |  |  | $ 338

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