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Pension, Savings, and Other Employee Benefits - Pension Plan and Savings Plan (Details) |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Nov. 20, 2017 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2016 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Nov. 30, 2017 pension_plan
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Contributions to qualified plans |  |  |  | $ 353,000 | $ 5,100,000 | $ 165,000,000 |
Pension Benefits |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Employer contributions |  |  |  | 3,948,000 | 4,789,000 |  |
Defined benefit plan, benefit obligation |  | $ 840,884,000 |  | 765,309,000 | 840,884,000 | 804,542,000 |
Fair value of plan assets |  | 811,244,000 |  | 730,953,000 | 811,244,000 | 778,872,000 |
Capital Bank Financial Corporation |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Benefit obligation, business combination | $ 18,500,000 |  |  |  |  |  |
Plan assets, business combination | $ 13,200,000 |  |  |  |  |  |
Qualified pension |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Contributions to qualified plans |  |  |  | $ 0 | 0 |  |
Qualified pension | Pension Benefits |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Estimated social security benefits age |  |  |  | 65 years |  |  |
Contributions to qualified plans |  |  | $ 165,000,000 |  | 0 |  |
Qualified pension | Capital Bank Financial Corporation | Pension Benefits |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Number of additional qualified pension plans | pension_plan |  |  |  |  |  |  | 2
Employer contributions |  | 5,100,000 |  |  |  |  |
Defined benefit plan, benefit obligation |  | 18,700,000 |  | $ 17,100,000 | 18,700,000 |  |
Fair value of plan assets |  | $ 18,600,000 |  | 16,500,000 | 18,600,000 |  |
Nonqualified pension | Pension Benefits |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Employer contributions |  |  |  | 5,800,000 |  |  |
Expected pension contribution |  |  |  | 5,200,000 |  |  |
Flexible Benefits Contribution |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Employer contributions |  |  |  | $ 29,300,000 | $ 23,000,000 | $ 21,600,000 |
Savings Plan |  |  |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |
Employer investment in qualified defined contribution plan |  |  |  | 100.00% |  |  |
Maximum percent of employee pre-tax contributions that may be matched by the Company (percent) |  |  |  | 6.00% |  |  |

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