Non Purchased Loans - Loan Portfolio by Risk-rating Grades (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 9,083,129	$ 7,620,322
Commercial, financial, agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,295,912	1,039,393
Real estate – construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	740,668	633,389
Real estate – 1-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,795,343	2,343,721
Real estate – commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	4,051,509	3,427,530
Installment loans to individuals
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	199,697	176,289
Non-Purchased
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	6,389,712	5,588,556
Non-Purchased | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	3,829,377	3,378,267
Non-Purchased | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	3,705,961	3,277,755
Non-Purchased | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	79,882	63,091
Non-Purchased | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	43,534	37,421
Non-Purchased | Commercial, financial, agricultural | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	641,102	570,841
Non-Purchased | Commercial, financial, agricultural | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	615,803	554,943
Non-Purchased | Commercial, financial, agricultural | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	18,326	11,496
Non-Purchased | Commercial, financial, agricultural | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	6,973	4,402
Non-Purchased | Real estate – construction | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	568,968	484,241
Non-Purchased | Real estate – construction | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	558,494	483,498
Non-Purchased | Real estate – construction | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,317	662
Non-Purchased | Real estate – construction | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	8,157	81
Non-Purchased | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	330,484	263,845
Non-Purchased | Real estate – 1-4 family mortgage | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	321,564	254,643
Non-Purchased | Real estate – 1-4 family mortgage | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	4,660	505
Non-Purchased | Real estate – 1-4 family mortgage | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	4,260	8,697
Non-Purchased | Real estate – commercial mortgage | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,288,823	2,058,419
Non-Purchased | Real estate – commercial mortgage | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,210,100	1,983,750
Non-Purchased | Real estate – commercial mortgage | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	54,579	50,428
Non-Purchased | Real estate – commercial mortgage | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	24,144	24,241
Non-Purchased | Installment loans to individuals | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	921
Non-Purchased | Installment loans to individuals | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	921
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