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EMPLOYEE BENEFIT PLANS - Narrative (Details) | 12 Months Ended
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Dec. 31, 2025 USD ($) plan shares | Dec. 31, 2024 USD ($) shares | Dec. 31, 2023 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of defined benefit pension plans | plan | 3 |  |
Defined benefit plan, accumulated benefit obligation | $ 1,110,000,000 | $ 1,090,000,000.00 |
Defined contribution plan, cost | $ 174,000,000 | 165,000,000 | $ 114,000,000
Deferred compensation arrangement with individual, maximum contractual term | 10 years |  |
Accrued personnel cost | $ 659,000,000 | 705,000,000 |
Pension Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Pension and other retirement plan assets | 685,000,000 | 557,000,000 |
Employer contributions | $ 6,000,000 | $ 6,000,000 |
Interest crediting rate | 4.50% | 4.00% | 4.25%
Discount rate | 5.57% | 5.69% |
Benefit costs related to Postretirement Plans | $ (23,000,000) | $ (22,000,000) | $ (15,000,000)
SVBB |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Pension and other retirement plan assets | $ 0 |  |
RSUs |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
RSUs vesting period | 3 years |  |
RSUs outstanding (in shares) | shares | 0 | 20,255 |
RSUs grants (in shares) | shares | 0 | 0 |
RSUs vested and settled in period, fair value |  | $ 31,000,000 |
BancShares Pension Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Calculation of postretirement benefits, term | 10 years | 10 years |
Bancorporation Pension Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Vesting period | 5 years | 5 years |
Number of consecutive years | 5 years | 5 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 | 4.80% | 4.30% |
401(k) Contribution Plan |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Number of defined benefit pension plans | 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 Legacy 401(k) Plan | Defined Contribution Plan, Employer Matching Contribution, First Portion |  |  |
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% |  |
FCB Legacy 401(k) Plan | Defined Contribution Plan, Employer Matching Contribution, Next Portion |  |  |
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% |  |
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% |  |
FCB 401(k) Plan | Defined Contribution Plan, Employer Matching Contribution, Minimum Contribution |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Defined contribution plan, employer matching contribution, percent of employees' gross pay | 3.00% |  |

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