Non Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Loan portfolio by risk-rating grades
Loans, net of unearned income	$ 9,313,784	$ 9,083,129
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	1,328,560	1,295,912
Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	816,695	740,668
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,797,633	2,795,343
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	4,022,375	4,051,509
Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	348,521	199,697
Non purchased loans and leases
Loan portfolio by risk-rating grades
Loans, net of unearned income	7,031,818	6,389,712
Non purchased loans and leases | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Loans, net of unearned income	4,277,183	3,829,377
Non purchased loans and leases | Internal Noninvestment Grade | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	767,592	641,102
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	702,949	568,968
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	327,404	330,484
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,479,210	2,288,823
Non purchased loans and leases | Internal Noninvestment Grade | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	28	0
Non purchased loans and leases | Pass
Loan portfolio by risk-rating grades
Loans, net of unearned income	4,173,682	3,705,961
Non purchased loans and leases | Pass | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	742,438	615,803
Non purchased loans and leases | Pass | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	691,112	558,494
Non purchased loans and leases | Pass | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	320,874	321,564
Non purchased loans and leases | Pass | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,419,230	2,210,100
Non purchased loans and leases | Pass | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	28	0
Non purchased loans and leases | Watch
Loan portfolio by risk-rating grades
Loans, net of unearned income	52,973	79,882
Non purchased loans and leases | Watch | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	12,351	18,326
Non purchased loans and leases | Watch | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,923	2,317
Non purchased loans and leases | Watch | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,520	4,660
Non purchased loans and leases | Watch | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	34,179	54,579
Non purchased loans and leases | Watch | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	0	0
Non purchased loans and leases | Substandard
Loan portfolio by risk-rating grades
Loans, net of unearned income	50,528	43,534
Non purchased loans and leases | Substandard | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	12,803	6,973
Non purchased loans and leases | Substandard | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	8,914	8,157
Non purchased loans and leases | Substandard | Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,010	4,260
Non purchased loans and leases | Substandard | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	25,801	24,144
Non purchased loans and leases | Substandard | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	$ 0	$ 0