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