Non Purchased Loans - Loan Portfolio by Risk-Rating Grades (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Loan portfolio by risk-rating grades
Loans, net of unearned income	$ 9,054,654	$ 9,083,129
Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	1,305,076	1,295,912
Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	781,531	740,668
Real estate – 1-4 family mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,765,472	2,795,343
Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	4,017,969	4,051,509
Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	184,606	199,697
Non purchased loans and leases
Loan portfolio by risk-rating grades
Loans, net of unearned income	6,704,288	6,389,712
Non purchased loans and leases | Internal Noninvestment Grade
Loan portfolio by risk-rating grades
Loans, net of unearned income	4,120,021	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	723,362	641,102
Non purchased loans and leases | Internal Noninvestment Grade | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	652,762	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	329,636	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	2,414,231	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	30	0
Non purchased loans and leases | Pass
Loan portfolio by risk-rating grades
Loans, net of unearned income	3,989,179	3,705,961
Non purchased loans and leases | Pass | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	695,794	615,803
Non purchased loans and leases | Pass | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	641,131	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	321,905	321,564
Non purchased loans and leases | Pass | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,330,319	2,210,100
Non purchased loans and leases | Pass | Installment loans to individuals
Loan portfolio by risk-rating grades
Loans, net of unearned income	30	0
Non purchased loans and leases | Watch
Loan portfolio by risk-rating grades
Loans, net of unearned income	81,882	79,882
Non purchased loans and leases | Watch | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	13,891	18,326
Non purchased loans and leases | Watch | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	2,759	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	4,561	4,660
Non purchased loans and leases | Watch | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	60,671	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	48,960	43,534
Non purchased loans and leases | Substandard | Commercial, financial, agricultural
Loan portfolio by risk-rating grades
Loans, net of unearned income	13,677	6,973
Non purchased loans and leases | Substandard | Real estate – construction
Loan portfolio by risk-rating grades
Loans, net of unearned income	8,872	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	3,170	4,260
Non purchased loans and leases | Substandard | Real estate – commercial mortgage
Loan portfolio by risk-rating grades
Loans, net of unearned income	23,241	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