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Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions |  | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
May 04, 2023 | Dec. 31, 2023 | Dec. 31, 2022 | Jan. 01, 2024 | Jan. 01, 2023 | Dec. 31, 2021 | Dec. 31, 2020
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration] |  | Other assets | Other assets |  |  |  |
Past due threshold period for nonaccrual status |  | 90 days |  |  |  |  |
Allowance for loan and lease losses |  | $ (773) | $ (685) |  |  | $ (670) | $ (963)
Retained earnings |  | $ 3,964 | 3,430 |  |  |  |
Accounting Standards Update 2022-02 |  |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Allowance for loan and lease losses |  |  | 6 |  | $ 6 |  |
Retained earnings |  |  |  |  | $ 4 |  |
Accounting Standards Update 2023-02 | Subsequent Event |  |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Retained earnings |  |  |  | $ 8 |  |  |
Furniture and Fixtures | Minimum |  |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Useful life of premises and equipment |  | 3 years |  |  |  |  |
Furniture and Fixtures | Maximum |  |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Useful life of premises and equipment |  | 15 years |  |  |  |  |
Building | Minimum |  |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Useful life of premises and equipment |  | 7 years |  |  |  |  |
Building | Maximum |  |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Useful life of premises and equipment |  | 45 years |  |  |  |  |
Deposit Bases |  |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Intangible assets, amortization period |  | 10 years |  |  |  |  |
Non-Accruing |  |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Accounts receivable, net |  | $ 13 | 12 |  |  |  |
TD Transaction |  |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Proceeds from merger contract termination | $ 200 |  |  |  |  |  |
Fee reimbursement received | $ 25 |  |  |  |  |  |
TD Merger Agreement |  |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |  |
Merger and integration expense |  | $ 51 | $ 87 |  |  |  |

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