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Pension, Savings, and Other Employee Benefits - Narrative (Details) $ in Millions | 1 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2017 USD ($) | Sep. 30, 2018 USD ($) pension_plan | Dec. 31, 2017 USD ($)
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% |
Pension plan | Qualified plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Estimated social security benefits age |  | 65 years |
Contributions | $ 5.1 |  |
Defined benefit plan, benefit obligation | 18.7 |  | $ 18.7
Defined benefit plan, fair value of plan assets | $ 18.6 |  | 18.6
Pension plan | Non-Qualified Plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions |  |  | $ 5.4
Expected pension contribution |  | $ 5.7 |
Pension plan | Capital Bank Financial Corporation | Qualified plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Number of additional qualified pension plans | pension_plan |  | 2 |

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