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Employee Benefit Plans - Narrative (Details) | 12 Months Ended |
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Dec. 31, 2023 USD ($) defined_benefit_plan | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Jan. 03, 2022 retirement_plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of defined benefit pension plans | defined_benefit_plan | 3 |  |  |
Interest crediting rate | 4.25% | 1.50% |  |
Pension assets | $ 474,000,000 | $ 343,000,000 |  |
Defined benefit plan, accumulated benefit obligation | 1,120,000,000 | 1,070,000,000.00 |  |
Defined contribution plan, cost | $ 114,000,000 | 55,000,000 | $ 36,000,000 |
Deferred compensation arrangement with individual, maximum contractual term | 10 years |  |  |
Incentive plan liabilities | $ 676,000,000 | 267,000,000 |  |
Postretirement Plans |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Employer contributions | 0 | 0 |  |
Number of retirement plans assumed | retirement_plan |  |  |  | 4
Plan termination | 0 | 27,000,000 |  |
BancShares Pension Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Employer contributions | $ 0 | 0 |  |
Vesting period | 5 years |  |  |
Number of consecutive years | 5 years |  |  |
Calculation of postretirement benefits, term | 10 years |  |  |
Bancorporation Pension Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Employer contributions | $ 0 | 0 |  |
Vesting period | 5 years |  |  |
Number of consecutive years | 5 years |  |  |
Calculation of postretirement benefits, term | 10 years |  |  |
CIT Pension Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Employer contributions | $ 0 | 0 |  |
Supplemental and Executive Retirement Plans |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Employer contributions | $ 0 | $ 0 |  |
Interest crediting rate | 3.90% | 1.90% |  |
401(k) Contribution Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Number of defined benefit pension plans | defined_benefit_plan | 2 |  |  |
FCB Legacy 401(k) Plan | Maximum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Defined contribution plan, employer matching contribution, percent of employees' gross pay | 4.50% |  |  |
FCB 401(k) Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Defined contribution plan, employer matching contribution, percent of match | 100.00% |  |  |
Defined contribution plan, employer non-elective contribution, vesting period | 3 years |  |  |
FCB 401(k) Plan | Maximum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Defined contribution plan, employer matching contribution, percent of employees' gross pay | 6.00% |  |  |
CIT 401(k) Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Defined contribution plan, employer matching contribution, percent of match | 100.00% |  |  |
CIT 401(k) Plan | Maximum |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Defined contribution plan, employer matching contribution, percent of employees' gross pay | 4.00% |  |  |
Defined Contribution Plan, Employer Matching Contribution, First Portion | FCB Legacy 401(k) Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Defined contribution plan, employer matching contribution, percent of match | 100.00% |  |  |
Defined contribution plan, employer matching contribution, percent of employees' gross pay | 3.00% |  |  |
Defined Contribution Plan, Employer Matching Contribution, Next Portion | FCB Legacy 401(k) Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Defined contribution plan, employer matching contribution, percent of match | 50.00% |  |  |
Defined contribution plan, employer matching contribution, percent of employees' gross pay | 3.00% |  |  |
Defined Contribution Plan, Employer Matching Contribution, Minimum Contribution | FCB 401(k) Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Defined contribution plan, employer matching contribution, percent of employees' gross pay | 3.00% |  |  |
RSUs |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
RSUs vesting period | 3 years |  |  |
RSUs vested and settled in period, fair value | $ 16,000,000 | $ 64,000,000 |  |

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