Loans and Allowance for Loan Losses - Nonaccrual loans by loan class (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	$ 676,716	$ 606,843
Nonaccrual With No Recorded Allowance	67,960
Commercial, Financial and Agricultural
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	323,881	268,288
Nonaccrual With No Recorded Allowance	34,522
Real estate – construction | Commercial Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	13,676	8,041
Nonaccrual With No Recorded Allowance	0
Commercial real estate – mortgage | Commercial Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	114,839	98,077
Nonaccrual With No Recorded Allowance	33,438
Residential real estate – mortgage | Residential Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	147,058	147,337
Nonaccrual With No Recorded Allowance	0
Equity lines of credit | Residential Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	33,354	38,113
Nonaccrual With No Recorded Allowance	0
Equity loans | Residential Real Estate
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	8,027	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	7,160	6,555
Nonaccrual With No Recorded Allowance	0
Consumer indirect | Consumer
Financing Receivable, Nonaccrual [Line Items]
Nonaccrual	28,721	$ 31,781
Nonaccrual With No Recorded Allowance	$ 0