Pension, Savings, and Other Employee Benefits - Pension Plan and Savings Plan (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Nov. 30, 2017 pension_plan
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified plans		$ 509	$ 353	$ 5,100
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, benefit obligation		835,874	765,309	840,884
Fair value of plan assets		826,070	730,953	811,244
Employer contributions		3,510	3,948
Qualified pension
Defined Benefit Plan Disclosure [Line Items]
Contributions to qualified plans		$ 0
Qualified pension | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Estimated social security benefits age		65 years
Qualified pension | Capital Bank Financial Corporation | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Number of additional qualified pension plans | pension_plan					2
Defined benefit plan, benefit obligation			17,100
Fair value of plan assets			16,500
Employer contributions	$ 500
Nonqualified pension | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 5,200
Expected pension contribution		5,200
Flexible Benefits Contribution
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 27,700	$ 29,300	$ 23,000
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%