Loans and Allowance for Loan Losses - Nonaccrual loans by loan class (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	$ 764,744	$ 606,843
Nonaccrual With No Recorded Allowance	142,105
Commercial, Financial and Agricultural
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	389,615	268,288
Nonaccrual With No Recorded Allowance	100,678
Real estate – construction | Commercial Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	13,317	8,041
Nonaccrual With No Recorded Allowance	5,836
Commercial real estate – mortgage | Commercial Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	117,213	98,077
Nonaccrual With No Recorded Allowance	35,591
Residential real estate – mortgage | Residential Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	169,387	147,337
Nonaccrual With No Recorded Allowance	0
Equity lines of credit | Residential Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	34,915	38,113
Nonaccrual With No Recorded Allowance	0
Equity loans | Residential Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	8,457	8,651
Nonaccrual With No Recorded Allowance	0
Credit card | Consumer
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	0	0
Nonaccrual With No Recorded Allowance	0
Consumer direct | Consumer
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	7,624	6,555
Nonaccrual With No Recorded Allowance	0
Consumer indirect | Consumer
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	24,216	$ 31,781
Nonaccrual With No Recorded Allowance	$ 0