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
Purchased
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,101,664	2,693,417
Purchased | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,385,653	1,852,090
Purchased | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,296,372	1,746,683
Purchased | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	64,291	84,250
Purchased | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	24,990	21,157
Purchased | Commercial, financial, agricultural | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	272,146	369,748
Purchased | Commercial, financial, agricultural | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	259,760	333,147
Purchased | Commercial, financial, agricultural | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	7,166	33,857
Purchased | Commercial, financial, agricultural | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	5,220	2,744
Purchased | Real estate – construction | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	48,994	101,964
Purchased | Real estate – construction | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	48,994	101,122
Purchased | Real estate – construction | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Purchased | Real estate – construction | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	842
Purchased | Real estate – 1-4 family mortgage | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	82,831	128,806
Purchased | Real estate – 1-4 family mortgage | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	78,105	113,874
Purchased | Real estate – 1-4 family mortgage | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	791	7,347
Purchased | Real estate – 1-4 family mortgage | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	3,935	7,585
Purchased | Real estate – commercial mortgage | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	981,682	1,251,570
Purchased | Real estate – commercial mortgage | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	909,513	1,198,540
Purchased | Real estate – commercial mortgage | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	56,334	43,046
Purchased | Real estate – commercial mortgage | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	15,835	9,984
Purchased | Installment loans to individuals | Internal Noninvestment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	2
Purchased | Installment loans to individuals | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Purchased | Installment loans to individuals | Watch
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Purchased | Installment loans to individuals | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 0	$ 2