Loans and Allowance for Loan Losses - Nonaccrual loans by loan class (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	$ 1,233,040	$ 606,843
Nonaccrual With No Recorded Allowance	95,080
Commercial, Financial and Agricultural
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	660,254	268,288
Nonaccrual With No Recorded Allowance	67,309
Real estate – construction | Commercial Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	12,614	8,041
Nonaccrual With No Recorded Allowance	0
Commercial real estate – mortgage | Commercial Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	275,668	98,077
Nonaccrual With No Recorded Allowance	27,771
Residential real estate – mortgage | Residential Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	204,442	147,337
Nonaccrual With No Recorded Allowance	0
Equity lines of credit | Residential Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	37,216	38,113
Nonaccrual With No Recorded Allowance	0
Equity loans | Residential Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	8,758	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	9,134	6,555
Nonaccrual With No Recorded Allowance	0
Consumer indirect | Consumer
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	24,954	$ 31,781
Nonaccrual With No Recorded Allowance	$ 0