Non Purchased Loans - Loan Portfolio by Risk-rating Grades (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 9,689,638	$ 9,083,129
Commercial, financial, agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,367,972	1,295,912
Real estate – construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	826,483	740,668
Real estate – 1-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,866,613	2,795,343
Real estate – commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	4,244,265	4,051,509
Installment loans to individuals
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	384,305	199,697
Non-Purchased
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	7,587,974	6,389,712
Non-Purchased | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	4,654,842	3,829,377
Non-Purchased | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	4,555,462	3,705,961
Non-Purchased | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	48,173	79,882
Non-Purchased | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	51,207	43,534
Non-Purchased | Commercial, financial, agricultural | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	803,462	641,102
Non-Purchased | Commercial, financial, agricultural | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	779,798	615,803
Non-Purchased | Commercial, financial, agricultural | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	11,949	18,326
Non-Purchased | Commercial, financial, agricultural | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	11,715	6,973
Non-Purchased | Real estate – construction | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	708,660	568,968
Non-Purchased | Real estate – construction | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	698,950	558,494
Non-Purchased | Real estate – construction | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	501	2,317
Non-Purchased | Real estate – construction | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	9,209	8,157
Non-Purchased | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	346,507	330,484
Non-Purchased | Real estate – 1-4 family mortgage | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	339,079	321,564
Non-Purchased | Real estate – 1-4 family mortgage | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	3,856	4,660
Non-Purchased | Real estate – 1-4 family mortgage | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	3,572	4,260
Non-Purchased | Real estate – commercial mortgage | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,796,207	2,288,823
Non-Purchased | Real estate – commercial mortgage | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,737,629	2,210,100
Non-Purchased | Real estate – commercial mortgage | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	31,867	54,579
Non-Purchased | Real estate – commercial mortgage | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	26,711	24,144
Non-Purchased | Installment loans to individuals | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	6	0
Non-Purchased | Installment loans to individuals | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	6	0
Non-Purchased | Installment loans to individuals | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Non-Purchased | Installment loans to individuals | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0	$ 0